Things to think about:
PTO's are not all interchangable, the internal driven gear needs to match the host transmission. The PTO off of a Clark transmission will look the same (SAE 6 Bolt), but won't work on the GM transmissions.
Motor mounts...you existing setup has the transmission hanging off of the bellhousing with four motor mount points (2 up front and 2 at the bellhousing). The modern SBC will have 2 side mounts and one on the transmission tailshaft. Make sure you have a plan before you start buying stuff as it might not bolt right in.
If you rebuild a 235/361 and the existing SM420 you'll have years and years of dependable service. They are simple to work on when needed and parts can be mailordered overnight from lots of vendors.
Mike B
