The bellhousing patterns are very different. You can use any 235/261 full pressure, or the 228/248/270/302 (best choice) as a virual bolt in using the existing bellhousing. The Chevy front mounts are different due to the length difference in the engines (GMC is longer).
A later 6 can be made to fit, but will require fabbing up side mounts for the block, and the bellhousing will have to be swapped out for a Chevy V8 bellhousing from '55-'66 with the crossmember mounts.
