@Johnny Dart
I heartily recommend the book, Performance Automotive Engine Math on Amazon.
Check the port runner size for the manifold/heads, they should match. I suspect that you already have a decent top end, and you could probably get away with just a shortbus.
Block, I meant short...
You are locked in with your transmission and the OD helps.
You are kinda locked in with your exhaust, heads, compression ratio and cam.
You are less locked in with carb and intake manifold.
Build everything based off the cam. You pick your cam based on the performance you want from the engine...
I don't remember, i did my research a while back, for MY application.
68 fastback, s-60 rear end (i don't remember offhand axle housing diameter), tire size, 002/003 leafs on spring sliders and solid bushings.
I love the Arai Quantum series for my lumpy head.
Just have to lose another 20 pounds and my wife says she'll buy me a motorcycle.
Having the flu is going a long way towards making that happen.
https://www.araiamericas.com/node/128