There are several possibilities with the 250, and some up and down side considerations when it comes to replacing your 235. It won't bolt in without a bellhousing change, and the fabrication of a crossmember or two. The rear of the 250 block has the same bolt pattern as a Chevy V-8, so a bellhousing for one of the old 265's or 283's will be similar, but not identical to the way your bellhousing mounts. It might be possible to modify your existing crossmember to accept the early V-8 bellhousing, which has provisions for a hydraulic clutch system on some models. The forward mounts are about mid-engine, which will require fabricating some mounting brackets.
The pistons in the 250 are dish-top for proper compression for unleaded regular gas. They can be replaced with 307 V-8 pistons which are flattops, for higher compression and performance. The size, weight, wrist pin, etc. of the 307 piston is identical to the 250, the only difference is the dome shape. Late-model 250 cylinder heads with the built-in intake manifold are a POS, so try to use the earlier head with the removable intake. Lots of work to make the swap, but parts are a little more available, and it's a newer design engine. A good-condition 261 would be an easier, more powerful swap.
Jerry
