Dear Josh;
Your plan use the 1956 261" engine is a good one and the best all around for keeping the "original look", & it bolts right in too.
There's no way to really tell how much extra power you can/will get as no 'dyno test' were done way back then.
An 'educated guess' would be around 50 Hp. more 261 vs 235 if you raised the compression & added a mild 3/4 cam, 75+/-.
If the 270 GMC engines are good, I'm sure there's a buyer here.
Good luck,
