I'm not sure about the R3, but on the Ritter block I had to make a few small adjustments to the ds header (TTI 1 7/8)
Where the cylinder head meets the block, the block is wider than the head, had to tweak a couple tubes there, also the bottom row of head studs interfered in a couple spots.
Resulted in keeping the header away from the head just slightly.
It was really close to fitting, had I not mocked it up on the engine stand, I would have had a really tough time finding the interference.
This was in an a-body.
Indy heads on aluminum Ritter block.
If a beveled spacer would have fit it could have moved
the header out a bit to clear the studs and maybe some
of the other issues
EDIT
Or cut about 7/8 of the way through the tubes at the
flange and bow them out and reweld them
