Stick with the 250. It takes less work to get more power out of the 250. Many parts are interchangeable with the SBC. Valve train is far better. Ever try to get non-cast rockers for a 261? My friend tried and finally gave up. The 250 is a better engine using 60's technology versus the 261 use WWII era technology. Chevy had a hard time justifying changes to improve their line of sixes in the 50's, one of their major downfalls.
Now, if you want a real engine, follow tylers lead and go with the 292.
