Need some advice on choosing an engine

On behalf of the crew here, your welcome. LOL!
Since your starting from scratch I'd go big block. Mainly cause of head choice, hard to pass up on the trick flow heads.

Either 383 or 400 for 434 to 480 cid or 440 for 480-512 cid.
I actually, really like this thought process. Even if you just grabbed a 383 and used a RB crank, that would be 426 cubes? (Correction sought) Or a stock 400 or with a RB crank for 451 cubes.

The only drawbacks to the big block are washed away by the pros over the cons.
The big block is heavier and uses the heavier 727 which requires more power to operate.
The engine itself is heavier. By a fare bit. They can be lightened up to a small blocks weight.

However, even at an equal cube face off of a stroke small block, AKA, 408 vs a .030 overbored 400, the available heads with larger valve sizes is a great plus, (Edelbrock head vs Edelbrock head for an example) , header tubes start larger. This all will create more power over the small block.

I don't know what you'll end up doing but have fun at it and post it up!