Have you tried finding a running 235 or 261 to drop in? It might be your cheapest way to go.
I had bought a running 235 that I decided to tear down. Something just didn't feel right. What I found was a driveway overhaul. Very little ridge, less that .010. but the rods were stretched, it showed on the crank. Popped the pistons, fairly new rings. Then I got out the snap gauges to check the bores. I had one cylinder that was so far out of round that it was within .005 of having to go to .080 over instead of .060.
If you do go for the later 250 block there will be a lot of work to make it fit. Chip is right about that, too.
