Wish my truck was closer to sea level, I bet the power gain from the 2100 foot level would be noticeable. If you had problems with manufacturing of the Weber Carb, don't hesitate to return it for a replacement. They are not real cheap, but rebuilding can be done in an hour and a half and they actually setup rather easy. Make sure everything is in sync with timing and stuff and you should have good luck. Usually with the weber when you add a header it needs to be re-jetted to be a little richer from prior the header install. You should get better mileage with a 32/36 also from the 38. The progressive opening on the 32/36 has the smaller primary and around town is an advantage compared to the 38.
If your pinging your engine, you need to raise your octane maybe.

The fuel/air mixture takes a split second to burn fully, so ignition should occur a few degrees before the piston rises fully to the top of the cylinder. We call this ignition timing and the engine computer precisely controls the instant at which the spark occurs. With pre-ignition, spontaneous combustion of the fuel/air mixture, before the spark ignites it, causes a more violent explosion. This is where the term pre-ignition originates. Ignition of the fuel and air occurs before the ignition system lights it.

When the fuel/air mixture explodes prematurely, the force created tries to drive the piston down the cylinder, while the crankshaft is forcing it up. A characteristic "pinging" is often heard, due to the forces in the engine. A pinging noise on acceleration is a symptom of pre-ignition. Left untreated pre-ignition can severely damage an engine. The excessive pressure developed can damage pistons and cause head gasket failure. Many things cause pre-ignition and correction always depends on finding and correcting the cause(s).

Are you using some cheap fuel station that puts crap additives and ethanol or whatever. I hate to see you grenade your engine or a head gasket from the problem. I hope you don't have any funky 4 or 3 pronged spark plugs either. Use the basic NGK is best bet. Anyway, here is a pretty informative page on that knock and pinging stuff. It should help get you diagnosed or on the right direction.

http://www.agcoauto.com/content/news/p2_articleid/60