Why dual exhuast vs. single?

[docflower]

I have a stock 72 c20 with a 350. The stock exhaust needs to be replaced to due to age and wear. I was thinking to put dual exhaust on to let it breath better, but my mechanic says not to do it. The truck is a daily driver 108K orig miles on it. Ramhorn exhaust manifold and quadrajet carb.

What are peoples thoughts (aside from cost) regarding putting dual exhaust on my truck? Aside from a bit of HP increase (which I dont really need) what changes would be seen? I have read from other threads there is also a consideration of having too little back pressure, would that be a problem?

Cheers!