Pinging help please

I have a 1991 Caprice wagon with the 5.7 350 tbi. The engine is out of a 93 Roadmaster with 45,000 miles on it. The car pings loudly when it warms up and is going up hills before it downshifts, and when you get on it and it doesn't downshift. It has new plugs, cap, wires, rotor the egr seems ok the timing was adjusted to factory specs, it has a new fuel pump and fuel filter. I have also tried to use premium which helps a little but it's still there. I had the valve covers off before it went in and it was as clean as a whistle. It also has a new speedo gear in the tranny because the Roadmaster had a different rear end. It has single exhaust cat delete that splits to dual with two Farm and Fleet Cherry bomb flowmaster mufflers not the red long ones but actual mufflers. It has the 700r4 tranny just like the stock 305 did. Would an o2 sensor cause it to ping. I work at an auto shop and even my boss is stumped by it. It also has horrible gas mileage with for a 5.7 tbi it only gets around 14mpg. Any help would be nice.
Thanks

Did you reprogram after the 305/350 swap? The advance tables for the 305 are a bit more aggressive than for the 350, even though the initial (base) timing is the same.

I put the computer from the roadmaster in my car that was for the 350 but it still had a different rear end and speedo gear could the 350 computer still have to be reprogrammed.

I had a car that pinged until the egr was replaced. Not a Buick 350, but I would see where might do it.

The engine in the buick was a gm 350 it is exactly the same. i might try to change the egr anyway. The problem doesn't really harm much it is just extremely annoying.