I thought you had the stage 6 intake for use with those heads on the RB block.
To use a "normal" intake you'll need the spacer plates.

Probably wouldn't help with the bottom end situation, but if the cam is only 252/257 @ .050, there is some power to be gained from going bigger.

Converter is 8", but what does it stall to on the brake with no chip?

68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads