A good aftermarket replacement for the 390 GT cam is a Comp Cams 252H high energy. It's basically the same grind as the 390 GT. If you want a little more bang for your buck, try the 268H High Energy cam. It's very streetable on a big block, but increases torque and high end performance without affecting vacuum operated accessories. I have the 268H on my '67 GT 500 428 PI and I like it a lot better than the original cam. Idle is just a bit lumpy, but throttle response and operation through 5000 rpm is fast and smooth. The only drawback I noticed over the 390 GT cam was a drop in gas milage (from 12 mpg to 10 mpg at 60 mph).