Howdy,
In terms of how the 235 vs 216 is mounted in the front, some earlier 235s, I think '55 1st edition back, mounted like the 216 with the single center 2 bolt mount. Things changed with the '55 second edition, though. I can't say for sure, but I believe that the V-8s and the 235/261s both used the same holes in the cross member for their mounts which were dual. The V-8 mounts were on the front of the block and the 235/261 mounts were on the side. The L6 mounts were kind a "stalk" affair with 2 rubber biscuits top and bottom and a rod in between.
The 250s side mounts sit farther back on the block than the 235, so custom mounts will have be built or bought and welded in place on the frame. My plan for my '56 is to measure the angle of the top of the original 235 head mounting surface relative to the frame then set up the 250 to exactly the same angle. That will keep all my drive line angles stock.
The 250 L6 also has a stronger bottom end due to it having eight main bearings. This should allow for a few more RPMs for the highway.
Also, in 1971 HP and torque ratings dropped almost 1/3 due to smog restrictions.
Les
