My single biggest improvement to my 65 van which had the 170, was to put in a bone stock 240 rebuilt. Can't ever go wrong with cubes.
240 is a bolt in isnt it? 250 is first gen 300 right?
250 is the Falcon family of sixes, the 240 and 300 were the same family. All had the SBF bell housing pattern, but the 250 design is the same as the 144/170/200 except for 1" taller block and a 4 bolt water pump instead of 3.