Engine runs bad in only 1st gear

[masor]

I've already searched the entire internet 2 times over to try to solve my problem so please don't use "search" as an answer.

So here's the deal. '75 C20. New 350 GM Goodwrench 290 Horse Crate Motor. When I installed the new motor I also used all new accessories; plugs, wires, HEI distributor, Edelbrock 650cfm carburetor (no choke), alternator, and pretty much everything else you can think of. No smog or emissions equipment either.

Timed it at 7* as my starting point and it would barely run. Turns out the timing tab marks didn't match the marks on the balancer. So I found TDC and made a mark on the balancer according to the "new TDC". Timed it back at 7* and it ran alright. Bumped it up to 12* and it started to ping. Backed it off to 10* and its running better with no pinging.

Now the new problem is that it wont run right in 1st gear. (SM465 trans, not low gear, 1st gear). Anything above 2,000 RPM whether I'm accelerating or holding a steady throttle, the engine will sputter, miss, and backfire. Now when I shift into second gear, it runs perfect and I can rev the engine as high as I want.

A friend of mine said to bump up the timing, but I know it will start pinging again. Someone else said it's not advancing right, and the advancing curve should be changed with new springs for the mechanical advance. Someone else said to take off the air regulator flap for the secondaries on the carburetor. At this point I've tried about everything my friends have suggested. I need more suggestions for what my problem could be. Anything helps, thanks.