I think the 51 passenger had an odd 'side' motor mount set up, check the other motor to see if it has the same cast in bosses. Or I think you can use the center foreward older style mount by welding a plate to the crossmember. Also that later style 235 has a low water pump which will have to be modified to fit.

It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed