Brian,
The response you received from vwnate1 is correct. At a minimum, you will need to fabricate motor mounts etc.
The combination you're asking about is a good set-up. I had an early 70's 250, stock 4-speed, and torque tube rear-end in a 52 that I recently sold.
It would run circles around my 216 and "early" 235. I also liked it because it came with hardened valve seats (for unleaded gas) and a stock HEI ignition system.
I'd weigh your abilities to fabricate the mounts etc. and the cost of the engine your looking at to what you find looking for a 235.
I don't imagine you'll be that far apart $$ wise, as most people that have the 250's are generally looking to practically give them away as well.
The 235 is probably gonna require that you either fabricate new motor mounts or drill out the holes to front mount it. Then you still have to work with clearance issues on the water pump.
Either way, it can be done, and it shouldn't be that difficult.
