It's a long way from a direct bolt in. Really it'll be the same amount of work involved as a small (or big) block v8 swap. Might as well go for the larger more powerful engine, unless you just really want to go with an inline.

If definitely want an inline in your '55 then go for the 250. I'd scrap the head, as it's certain to be the integral type (if it's original). Beef up the camming and intake and exhaust while you're at it and you won't be disappointed.

A healthy stock 250 will turn 3000 sustained (cruising) rpms easy. A 235 won't. With a little bottom end balance work 5000+ peak rpms is easily attainable.

-ftyler