what fits easy is any 283 or 327 - apparently some 350's have the mount holes in the front of the block, but I've never seen one - they do have the bosses cast on the block tho, so could be drilled - the I6 and V8 brackets that bolt to the engine [smallish right angle piece] are the same shape/size, but the holes are a bit different - can be adapted - the peg mounts for the 6 are shorter than the V8 ones, can be lengthened - the V8 trucks had special mount pads riveted on to the back of the front cross member, that are not on the 6 cyl trucks - the complete mounts are available
as a kit from most suppliers - need a V8 bellhousing
I added the mount-kit frame brackets on my 58 and rebuilt a 283 for it - also later saginaw all syncro 3 spd just needs a fine tooth yoke to fit right in - the correct throttle linkage stuff is available from someone [Jim Carter parts?]
easiest conversion, not too spendy = 283 w/ repro mount kit, original 3 speed - later 6's mount different, 292 length will need modifications
Bill