On my Caterpillar powered Chevy I have a#3 SAE Adapter, and I used a bellhousing out of a Kenworth. The index hole is the same size as the early GM NV4500. Also the same as the GM NP435 I have. My flywheel is custom though. We simply redrilled the Kenworth bell to match the NV4500. Also drilled patterns for other transmissions that I thought I might try if the NV4500 didn't hold up. It held up, so didn't use the other holes.the SAE are all med duty trans ( Allison ,Spicer ,rockwell ,fuller ).