340 carb size

What is the compression ? Maybe the rings are worn out. Did you degree the cam ? The spring for the secondaries might be too stiff.

I run two 500 cfm Carters on a old eddy 273 intake on my 340 with 3.91 gears. Pulls really nice, the 10 inch 3,000 stall converter is a big help as well. The 340 needs rpm's