305 problem

I just recently swapped a stock mid 80s 305 out of a motor home into my 83 Malibu. The motor ran when it was pulled (saw it running) and then it sat in a heated and closed shop for about 3 months. The motor and tranny are now in and buttoned up and I have used a V8 wiring harness from an 81 Malibu. The issue you I'm having is it will only start and run at full throttle and then it runs like a piece of junk. I pulled the old Qjet off and put on a freshly rebuilt one (done by professional) because I suspected it might be flooding itself but it's still doing the same thing. I know for a fact it isn't timing because it hasn't changed since it was in the RV. And I can't find the source of a vacuum leak to cause a problem like that. When running with either carbs you can smell that it's rich. Haven't really played with the mixture screws yet. I'm kind've at a loss right now, anyone have any suggestions?

Is it possible your exhaust system is blocked?? That is the only thing I can think of that would be different.

Do you have or can you borrow a vacuum gauge?
Have you inspected each piece of vacuum hose?

Check valve for brake booster

Fuel pressure is too high.
Remove the electric pump in the tank, and install a good ol' reliable mechanical pump.

I just put a brand new exhaust system and headers on so I don't think it'd be blocked.

I don't have access to a vacuum gauge but all vacuum lines are brand new and none of them are leaking

The check valve seems to be fine

And I don't have an electric pump, it's all mechanical

was wiring harness from a fuel injected car?????????