Manifold or Headers, H or X pipe, muffler brand

[FatherandSon]

I just installed new GMPP 350/290hp in my stock 69 C20 daily driver. Right now it has the stock manifold to single pipe/muffler exhaust. GMPP installation specs say best exhaust for engine is long tube headers with 1 3/4" primary & 3" collector to 2 1/2" pipe w/H pipe. My goal like most is to make it sound nice & improve performance but not spend a lot. Unfortunately, I know little about exhaust systems.

Here are my concerns/questions:
1. Most headers available are 1 1/2" tube. A Super Chevy article indicated a 1 5/8" tube is best & can produce up to 16 hp but larger tubes drain low end torque. GMPP says 1 3/4" is best. Does the 1/4" or 1/8" make much difference?
2. A lot of posts on this board indicate a strong preference for stock manifold over problems w/headers. Will I notice the increase in hp w/headers or be wasting my money?
3. Several posts indicate a preference for X pipes over H pipes for sound quality but some indicate this may reduce low end torque by reducing back pressure. Which is best?
4. Which mufflers? Are Magnaflo & Flowmaster worth the price or is there a quality lower cost option?