The 235 would be the easiest to swap in but be aware that there will be several issues to contend with. Bellhousing, water pump/fan, and front mount plate are a few that come to mind. The better 235 is the 58-62 but nothing wrong with the 54-57. It never bothers me to see the oil gauge pegged out all the time but others prefer to use the 60# gauge. This swap has been done many times so both parts and info can be found. The bellhousing bolt pattern on the back of the 250 is completely different but does take any trans/bellhousing for the 4,6, or V8 engine. With the gas price sky rocketing many are going this route along with an open driveline so shallower gears or an overdrive of some sorts can be used. I kept my 37 Buick original for the 30 years I've had it but with the last time of $4 gas and it having the optional 4.44 "city" gears I put an open drive in it with 2.73 gears. The straight 8 is a torque monster so the low end didn't suffer and it is quieter on the highway.
