Looking for advice on engine upgrades

[E.D.75GMC]

I'm looking to get more power out of truck. It's a 1975 GMC 3/4 ton with the original 350 4bbl (has 4-bolt mains) and TH350. As it sits now it's in excellent running condition with no leaks, and the inside of the engine is very clean and it makes no noise. I have a set of SBC headers hanging in my garage that came with an 81 Blazer I used to own. I've been mulling over putting the headers on, and while I'm at it I may as well do an intake manifold as well as a cam and lifter kit. Wondering what people's opinions are on good parts to order. I did the timing chain and gear set back in August. Also, the carb that is currently on it is a rebuilt quadrajet that's only 16 months old so preferably I would like to keep that carb.