The S&S lifter blocks for the 4" bore (and larger) have the same footprint as the OEM blocks. They all use the same gasket, which fits right back to 1948. (Front and rear are different). The difference in the lifter blocks for the larger bore motors (which have the SA crankcases with the .250" camchest offset), is that the counterbores for the pushrod tube location are machined at a different angle to allow for the offset.
I don't work on Ultimas, so I couldn't comment on these.
There are only two reasons that this can happen on motor that hasn't floated the valves. Lifter clearances not machined deep enough for the cam being used (most likely), or the lifter block decks on the crankcase machined too deep (uncommon, but you might have rogue cases). This would move the whole of the lifter block closer to the camshaft. Also, some camshafts achieve their higher lift by reducing the base circle (which keeps the lifter rollers away from the blocks), whilst others will have an increased lobe height (which moves the rollers closer to the block). So a brand X cam may have issues, whereas a brand Y of similar lift may not.
Considering where you already are with this, I would pull the cam cover and dry build it (lifter blocks, gaskets, and lifters only, not pushrods). Rotate the engine and check for clearance issues. The outboard end of the cam will be unsupported, but as there will be no load on it, you can steady it against the pinion and breather gears.