I'm in the middle of trying to break in the cams on two different RB motors, one is a 1976 440 motor home block and the other one is a 1972 413 C.I. H.D. truck industrial block.
I borrow a friends engine run in stand to do this and he used a piece 7/16 I.D. thick wall tubing to use a single 7/16 bolt to go through the upper motor mount lugs, if fit fine on a 400 motor I ran on this stand two months ago but it won't fit on either of these two blocks due to it being to long for them on the passenger side
I'm going to use a grinder to reduce the over all length of the tube on that side, the other side fits fine
I did measure two 400 blocks and they are a lot wider between the lugs than either of the RB blocks
Any one else run into this type problems on the B and RB blocks

They talk about that here, I think this is what you are looking for.
CLICK HERE

Huh, I read that 440Source info but had forgotten about it. If I remember correctly, it has an impact when you use older motor mounts. Going to have to take a look at that today !! Thanks for bringing it up Cab

The engine mounts will fit all the blocks, the O.D. remained the same
The I.D between the RB lugs got narrower, evidently, when Ma Mopar increased the thickness of them
All three 400 blocks I checked are a lot narrower I.D. than the two later cast RB blocks are

The ears are thinker on 75 and later blocks but the mount goes on the back side of the ears so that does effect the mount. Just need longer bolts
On my 72 SSP I removed a 68 383 and installed a 75 440 using the same mounts with longer bolts.

I measured a 1968 440 block I have and it was right at .075 thinner on the passenger side upper front engine mount lugs than he two later RB blocks I listed I ended up having to grind a little over .075 off of the mounting tube to get it to fit in between the 1976 440 motor home block mounting lugs
Now to finish the plumbing, wiring, fuel system exhaust and ignition to hear this puppy roar