Well since you have a 250 you already have the "better" engine. Starting with the head I'd track down a 194 head. Already a compression booster there. Have it lump ported. Chamber relief will be require(I believe) to have 1.94 intake valves installed so look into that. Also 1.60 exhaust valves will be a good idea. Don't know if you shave the surface or not...Tom Lowe would be the one to ask...he has a Rocking 250 in his Chevelle wagon. Moving to the block....I'd do:
307 V8 flat top pistons.
Stock rods balanced and refitted with ARP studs and bolts. Or if you can afford a better custom set look into that as well.
A cam with around .550 lift will put you in the 300 hp rang. I think comp cams or one of Tom's cams has that lift.
Crankshaft balanced and ground.
Add a clifford or offy intake with headers and HEI you should see some 310 hp and 280 torque. Here's the video of a 250 similar to what I described.
https://www.youtube.com/watch?v=1p3-cAJrLCw
