How many miles did the first manifold go? Are you ok with it if it makes it that far again?
Back in the day when I was a 22re nut, I loved headers. I used the downey brand. The headers were good and didn't seem to leak or crack. I coupled them with a 2.5"(some say torque loss but not my experience) mandrel exhaust with a mild camshaft and it opened the truck up. The little beast gained 8-10mph up the passes.
I guess my point was make sure you complete the package or the headers will give you no gains. I'm not saying to buy cams, just do the whole exhaust. I'm not sold on intakes yet, they don't offer much in performance. Intakes do allow a more direct path for water though.
The 5v sounds mean with the Magnaflow, but it can get anoying. But I'm like Mario Andretti, I like to hear the horsepower(even if it's only 75).