Teach me about exhaust

[Kalums]

Ok gents... I want to learn something new about exhaust. I've got a 1971 GMC K2500 with a 350/TH350 +NP205... After 6 months of chasing my tail I've finally figured out ultimately/finally why I can not get my engine to run "right". After taking off the intake manifold to swap it and the carb out I found I had a bent push rod on cylinder(s) 7 AND 8 (along with other carnage) *sighs* long story short me thinks my engine needs a complete teardown and rebuild. OR I still have that GM TBI crate motor, (no one bought it yet) may just swap that in... *thinks* hhmmm. Either way, I'm going to keep the stock RAM exhaust headers( provided they fit) ... But why is my current exhaust 2" from the manifold through the "y" pipe to the muffler and then 2.25 to the exit behind the driver's side rear wheel? Taking sound out of the equation, why do alot of after market set ups/ replacements run 2.5" from the manifolds back? If I'm interested in torque vs "go fast" should I keep the replacement as close to my current setup as possible? Why does my current setup have the "cross pipe" going directly under my transmission? Doesn't that effectively cook the fluid? What benefit is duals? What harm is duals? Sorry for all the questions and thank you in advance.