Thank for all the advice guys. Meet a really reliable local builder here in the area and we are working a spec sheet for a 383. Now if I were to go bigger with a 400 0r 427, would I have to start looking at clearance, weight, frame concerns ant thing like that?
Have you changed your stated use for the car since your first post? Certainly ok if you did, just asking.
You may find yourself disappointed in the results if you 'spec' out any engine w/o serious considerations for the drive train (transmission, rear, tires). Just a head's up while trying to be helpful.
Just some of my opinions on a '60s full size Chevy 'street driver':
Keep the 'power range' similar to a GM producing engine (idle to 5000 rpm 'ish'). High rpms can make one go fast but puts higher stresses on components and reduces reliability.
Make low end torque a priority over max hp. An engine that can give you 400 lb ft of torque at around 2k rpm and maxes out at around 440 at around 3800 rpm will be a smooth, powerful, fast, reliable engine throughout the normal rpm range of street driving. Towards that goal, give displacement priority over high rpm. Such an engine will give you more flexibility in drive train choices.
You can go for more displacement, but a 383 built with today's components (not racing parts, just quality modern designs) can easily fit the recommendation above.
Nix the headers, more trouble than they a worth for an engine running idle to 5k rpm. GM never put headers on their production muscle cars; not even the HIGH revving Z28. Those cars were FAST. Would they be FASTER w/headers? Yes. But also more problematic.
Like I said, just my opinions offered for your consideration while trying to be helpful. Give them what ever weight you wish.