Not sure if its the Eagle crank having larger weights, or my block being thicker down there, but just checked the clearance to the machined releif area, only 0.005 to .015 clearance from web to web.
By reading some other posts and forums, people usually just drop it and start torquing things down.

How much end play, thrust bearing to the crank thrust flange, does it have now?

It has .006 end play, I used a stripped down angle grinder to grind in about .050 clearance for the counterweights to the block.
The closest point was right at the bore edge when the counterweight was vertical up in the block, only had about .005 clearance there,

I honestly do NOT know how much to little clearance is on the crank counterweights
I shoot for .030+ on the rotating crank and rod parts on what I can see and measure, look and learn
A lot more clearance is way better than a tiny bit to little
A good rotary hand grinder with good carbide bits is is your friend

They will probably have to cut the counterweights down when they balance the crank for today's lightweight rotating assemblys but still .005 sounds tight.

I had checked with the machine shop on that, they said they would prefer I cut the room out of the block, as they didn't think the eagle cranks had that much excess weight on them.
Getting in that area at the side of the bore was tough with a die grinder. I put a fresh 1/4" thick wheel on the angle grinder and just had to give it a few light zings, cast iron grinds really easy. The grinder wheel makes a nice radius cut there.
I had googled it and found people just dropping these cranks in with no mention of clearances whatsoever, guess they got lucky or the crank clearanced the block itself.

Is this on a 383-400 block? I remember years ago "Challenger340" did an article on putting a 440 crank in a 400 block to make a 451 and the taller counterweights of the 440 crank would usually hit in the area you were grinding on in the photos. On his build he would put a large chamfer on the crank counterweight to gain clearance in that area before the crank was balanced.

I would have the crank counterweights reduced. Even just a bevel on the edge where you need clearance.
Calculate your bobweight (estimate) from the parts you will use. Check with Eagle for the approximate bobweight that specific model crank comes with.
I think you will find a lot of weight will need to be removed from the crank.

For example:
Piston 564.0
.990x2.93x.185 Pin 178.5
Spiro locks 4.5
Rings 45.0
I-beam small end 247.0
Con rod big end 517.0
Con rod big end 517.0
Con rod bearing 52.0
Con rod bearing 52.0
Trapped oil 4.0
Bob Weight 2181

Is this a B block or RB?

Rb , 1977 originally a passenger car block by the looks of it.

Very timely for me to see this post last night after just coming in from the garage and discovering the same problem with a 4.00 Scat crank in a Ritter small block. The issue with this block looks to be the strong CNC radius at the wider main saddle to cylinder junction. Works like I'll be getting the grinder out this weekend.

Thank you very much for the info.

I think it is great to check things out and post the info, it's not like these are secrets or anything.
Posting things like that is a pretty thankless task but it helps others out.

I've tried to do that when I had the chance and years later had someone mention it,
but it tended to happen more often when information was actively archived instead of allowed to die
after 2 years as it is currently.

Maybe some of the perception that people take these out of the box and throw them together is caused
by folks not always giving credit to those who were hired to do the build and make all the decisions...
Or, even when credit is given, small details like this might not have been documented.
That's my fault for wasting time trying to guess stuff like that....there is no accounting for human nature.

It doesn't really add much to the conversation but I had tons of room in this World iron K1 4.500.
I think I eyeballed it somewhere near 1/4"/not worth measuring.

.050-.060 is preferred safe clearance. Always do the counterweights when you can, much better way. ( all depends on bob weights too)Also, if you just go all crazy with a die grinder and dont do a good job of working that down, and smoothing it all, every one of those marks is a potential stress riser, aiding to a crack down the road.If you can see a sand mark, or heavy gouge, its a stress riser.

From the Eagle website, it looks like the crank should be about 2500 gram bobweight before balancing.
If your actual parts produce a required bobweight around 2200-2300 grams, the crank will be full of holes from the balance shop. Unless you tell them to reduce the counterweight diameter.

It seems that shop doesn't know what they are talking about.

I've never used an Eagle crank so I don't know what sort of issues they have. Molnar cranks typically drop in and clear just fine as do SCAT fully machined cranks. I've had clearance problems in the past using Mopar cranks. Typically a 4.250 crank is going to have a low bobweight so the counterweights need to turned down anyway.

After you get the parts weighed to calculate the actual bobweight needed,
then you can estimate where to start with counterweight diameter. The shop will likely have to go back and forth from cutting some off the counterweights, check the bobweight, and cut some more. Then do final balancing with the regular drill method.

I expect starting with counterweights at 7.31" dia would be safe, assuming the parts require a bobweight below 2300 grams.
Then maybe try 7.25" dia.
This is my Callies 4.250" stroke crank, before cutting the counter weights down to around 7.14" dia (bobweight 2185 in a low deck with 6.535" rods).

All good info, no matter how many books I read, I always learn stuff from this forum... I plugged in what I could find for my parts, I used the rod weight percentage ratio for an 880 gram rod, my pistons were a little heavier at 192. I come out to around 2300.
I am guessing the shop would like to just drill the crank rather than turn it for clearance, I got the space now. This motor isn't going to see any real rpm, so I hope the metal removed isn't going to really affect its life span. But I am armed with more knowledge for the next time.
If it all goes egg shaped, I guess I'll be buying a one of those new skookum blocks.

?????
https://www.skookumtough.com/

Skookum is Canadian for special.