Consider an SMR adapter, or designing something similar? I picked one up used for $200 a few weeks ago of off Craigslist. A few guys in the race section seem to believe it will work for a 512 build (minus the flex plate).

We race Ultrabell's with trans brakes all the time w/o issue. And you're worried about a conservative street car? That Ultrabell bolted on is every bit as good as the prehistoric sand casting. The bolts are for clamp load, not shear. It's clamp load that holds everything together. If its torqued you won't have an issue. If it bothers you, stud it. Good luck getting it dialed in on center with-in .005". If not, trimming a block for starter sounds a whole lot easier than what you have planned.
Doug

i have the ultra bell on my 512cid a518 68 charger so far it's good.

Thats pretty much a smoking deal on that! Retail is like $670.

The Ultrabell is SFI approved. I bought one years ago but have yet to install it. Maybe it'll go on the Imperial.

Don't underestimate the strength of the 727 case. I personally think it will handle any amount of power your engine has. High frequency cleans the weld so I wouldn't worry to much about that, just weld both sides and make tight joints. I've thought of this many times but never got around to trying it myself, good luck, got my vote!
Waiting for results!

I'm pretty sure the Torqueflite cases are die castings, same as the aluminum steering boxes. They're too smooth to be sand castings.

R.

It's gonna take a bit to get the jig made properly -- but I will most certainly post the results when I get that far.

Same here, got mine during that group-buy that happened on here a few years ago!

Just wondering if you ever completed this project, do you still have the Jigs or Pictures of them?

Thanks!

Ryan

You may consider using a A618/47RH. It can handle more power than the A518. I would assume you could weld on the ultrabell since it to is aluminum(along with the bolts). I believe the pump bolt pattern is the same between the 518 and 47 so the ultrabell would be an easier install. I see they make a adapter plate to install a ZF8 to a big block, That would be even better than any 518/47rh but would need a controller to run it.
link

Difference from a 518 to a 618, other than the bellhousing size, is the internals mainly in the clutch/steel count of the OD section, which can easily be built to 618/47R* spec... you would not want to run the diesel spec valvebody behind a gas engine .

Wouldn't a gear vendors OD be a better route????

it could be.
however, i priced out a JW Ultra bell [big block], a rebuild kit for the A518, a valvebody kit, and billet servo kits, and it came in roughly $1000.00 cheaper than a gear vendors unit.
i did not include any driveshaft work because one would be doing it [shortening or replacing the driveshaft] regardless of which way one went.
i also didn't include a converter, because of so many choices one could make.
the A518 also has a more favorable [in my opinion] overdrive ratio at .68 than the gear vendor's .072.
it [the A158] is also a shorter package overall, unless you have a "shorty" 727 to start with.
the A518 may require some tunnel mods, but the gear vendors may, or may not require some as well.
remember, this is just my opinion/observation.
your mileage will vary.