If you are going to rebuild a motor anyway, I would not go to the trouble of converting to a 250. I don't think its going to be enough more power to justify the conversion.
The 302 would be much more power for not a lot of conversion. The front mount is different and the motor is longer than a 235, but it looks very similar to the original, and parts from a GMC truck would allow you to use stock parts to move the radiator forward.
If you are going to convert to a later motor I'd go with the 292. You'll have some conversion issues. You'll need a different bellhousing that isn't going to fit your crossmember correctly and you'll need to build side mounts for the motor.
That's my two-cents. I hope its worth the trouble to read it all...
