The 261 head has lower compression but is otherwise identical in combustion chamber design to the 235 head. There are some extra water passage holes in the 261. A common modification is to run a 235 head on a 261 with the extra holes drilled.
If you are using new pistons then just design the pistons to up the compression and you will get what you need with the 261 head.
If you are building an all-out racing motor with custom pistons, cam, and head work, it makes more sense to use a 302. Its only really the stock parts that are more expensive, the machine work costs the same no matter what engine. The Jimmy head has a much better chamber design with side-by-side valves.
I picked up a good running 302 last year for under $300, so they are out there if you look. I'm going to replace my 261 with it.
