Actually I think the 474 Purple shaft was used in the 440 HP motors with a 10.2:1 compression (6-pak)

I do kind of agree with the previous post about the cam/converter/gearing not matched up correctly with the DQ.

My setup, as I am still building it, is a CH-28 Dual Quad with Dual 500 Eddy's. A .541 roller CompCams, 3000 B&M converter, 11:1 CR, and 3.55s with optional 3.91s if I wanna get cooky....Given this setup is in a 509 stroker, but I think you will definitely need more gear....

Since its a heavy car, maybe a CrossRam would better suit you with some AFBs?