The heat I'm referring to is "Engine Compartment Heat" which creates problems for you if you're cruising yer ride in hot weather . . . And . . You said you wanted A/C so I'm assuming you're not interested in an 11 second street ride . . . Just based on what you're starting with . .
Go to Summit and look at what they have to offer in a tube header. They should have an application that fits . . How good, that's up to you.
And . . If they don't have a GM G Body to BBC headers, I think I'd recommend that you get out your shoehorn and put the engine in the hole and see how much room you have left to play with before you order something that won't fit . .