You can find 261 short blocks and cores in junk yards or buy one from Langdons. The main difference between a 261 and a 235 is cylinder bore, one is 3 9/16 and the other is 3 1/2, or something like that. The later 261 also has a fully flowing full pressure oil filtration set up that none of the 235's came with.
Since the 235 head, crank, rods, cam, bearings, etc...all fit on a 261, if you've got the choice and plan on building, get a 261 block and the rest of it is easy to come by.
