Why is a spreader needed to get the sure-grip out of a dana ? Can the area needing the clearance not be ground ? I haven't dug into this yet so don't know the how's or why's.

It’s a delicate dance…. You have to spread the case to get the shims out… but not too much or you’ll bend it. That’s why we like the S60!

The case spread sets the bearing preload, you can get worn bearings out with big prybars and beat it back in with deadblow hammers.. new bearings will need a spreader for the most part to do it right.

Done a couple dozen gear swaps over the years and never used a spreader and never had an issue. A spreader would make life easier for sure in place of pry bars and a deadblow hammer. I made some set-up bearings years ago and it makes the job much easier. Spreaders are pretty cheap now on Ebay and Amazon

Gus

I don't use a case spreader, never have, never will
I remove the center section by using a pry bar gently to work it out a little at a time and then remove the bearing and shims with a bearing puller , I made some set up bearing like Gus uses and use them to set up the new gears and get the preload and back lash correct and then either press on the old bearing with the new shim stack or with the new bearings and races if their getting changed out with the new shims .
I'll use enough shims to get zero preload with no ring gear on the center section and then add another .012 to .015 additional shims for preload depending on which works the best
i use a plastic or leather mallet to tap in the center section, very carefully

When I removed the 4.10:1 ratio gears someone put in for the correct 3.54:1 gears, I noticed that the right bearing housing was missing a chip out of it.
Someone really forced the carrier into (or out of) the housing. It was in so tight that even with a spreader it was hard to remove. The carrier bearings were a bit discolored too from too much pre-load.
I did the setup bearings too, but they just get you close, once the real bearings are pressed on, it adds to the pre-load. Same with the pinion height, need to set it slightly low with the setup bearings so when the pressed on bearing expands the pinion come up to the correct height.
The pinion pre-load needs to be shimmed for the correct turning torque when there is at least 250 ft/lbs torque on the pinion nut, so you need a yoke holder and large torque wrench. A dial type in/lb torque wrench works good for measuring the pre-load.
Because the housing was slightly damaged where the carrier was forced in, I replaced the cap bolts with ARP studs too.
I used the carrier shims that go between the carrier and bearing. The clamshell style bearing puller works good to remove the bearings without damaging them.
On the pinion, I used the large diameter type shim between the case and bearing race for pinion height.
Pretty easy to have over $1,000 in tools to do a differential.

Well let me explain the dilemma then ...

Bought a very nice complete low milage '69 b-body dana at Carlisle. At some point it had some sort of traction bar (maybe ladder bars) mounts attached so there is evidence of the welds but nothing serious.

Plan A would be to seal up the breather, remove the brake parts and give it a very light sandblasting and prime and paint

Plan B would be complete disassembly and then narrow it a few inches.

Plan C sell it and buy a custom unit

The welding for the bars may have tweaked the housing, it's unknown history and If you want is narrower, plan C is like going to be the best and cheapest route
that low mileage may have been a 1/4 mile at a time

May be but not with the current 3.54's - which were swapped in for the 4.10's

I'm not worried about weakening from welds.

I'll put it up on Marketplace and test the waters.

I have found setup bearings to be dead accurate if you make them yourself and make them just fit as in slightly tap on and off, and use the same brand bearing to assemble. I am not a fan of beating carriers in with .012-.015 of shim so I have a spreader. Check out the Jantz engineering 2 part series on u-tube to see how to set up a dana fast. He made it so easy to get perfect patterns. Carl Jantz also has a u-tube on how to make setup bearings, and another showing gear patterns as you move the pinion in and out. Believe it or not I made a set of 3 dana setup bearings with one Dremel brand rough sanding roll, there are other ways but it took all of 5 minutes. Anyway whatever you decide Dana's are easy done the Jantz way.

I have a case spreader. Have used it a few times, over the years, to disassemble units that had never been apart from new. I have never put one back together so tight that I had to use a spreader.

I bent an 8 3/4 housing really badly drag racing it with ladder bars and coil overs, bad enough that I had to pound the 35 splines axles in with a BFH and a block of wood: tsk: It finally broke the welds off of the ladder bar mounts off of housing forcing me to see the bent right axle tube with it on jackstands: puke:
I replaced it with a cut down Dana 60 housing out of a pickup, I use two carpenter squares and a tape measure with the help of my wife to measure 15 inches out from the axle centers both forward and backwards to check for the housing being bent. That check can be done on jackstands also so you can measure all four ways, up, down, front and back of the housing :twocents

I would go with plan C especially if it's got the right numbers and dates for an A12 car, you should be able to get enough to buy an S60 setup as you want . Otherwise if it's really low miles and not ugly inside plan A will be fine.

I bought one of the Ebay spreaders, you only need to spread it no more than .020 ... per service manual ... or you risk damaging the housing ... yes you can pry it out and it back in but where is the metal burrs you are raising up going as you it back in ?

I was referring to TWEAKING as in bending of the axle tubes from the heat of the welds

I figured out long ago that most of the mechanic shops around here don’t buy the proper tools to do jobs properly. If they get a rear end that makes noise, then they just blame it on the gears and say “ Well some gears just make noise”. I bought my own gear depth setting tool long ago from Mark Williams. That makes setups an easy, fast and accurate job. I’ve never had a “ howler” yet. I’ve done two Dana’s without the case spreader and while it can be done, it’s just not the best way you can proceed. Especially with the cost of a spreader on eBay. I also used setup bearings on the 8 3/4s up until recently. I knew I had several Danas left to set up in my near future so I bought a clam shell bearing remover. Man that is slick. The setup bearings set up a little looser than new bearings. Now days I have very faith that all bearings made the same. You buy a Timken bearing now and you never know which foreign country it’s going to come from. USA bearings are getting harder to find. Back to your original question, though. Selling yours is an option but don’t expect to sell it fast or get good money for it. Shipping is a pain and expensive. Strange has a deal worked out with a shipping company and it cost much less than you can ship. I had a very rare nicely restored 72 Dana with all new parts from our own DoctorDiff. I like to have never sold it. I finally traded it off locally for about $1500 worth of parts.

i just bought a set of Timkin bearings to replace those in a Bridgeport mill.
they were made in Bangladesh...............

Can someone decipher these numbers for me please ...

That’s the date, with 8 meaning 1968. The B is for B body. There is another number called the BOM number. There is a chart floating around on the internet that can decipher that for you.

There should be more numbers in that area . , the BOM will start with a 6 , possibly there is a second date code.

That rear is too early for an A12 .