Bob--I run the 60303 in:
70 'Cuda
4 speed
3.55
850dp
906's
Eddy rpm
supposedely 10:1
18 initial, 34-36 total.

Cam installed at 106.5

Powerband comes on suddenly at 2000rpm, pulls hard. Haven't found the ceiling yet but its over 5600. Your's falls off at 4000? Somethings wrong.

Lunati will tell you that 104 is the min recommended install. I may actually re-do mine to 104.5 just to drop the p-band down a bit but by no means is it a dog on the bottom. I can fry my 275/50's at under 2500 with just a little bit of extra right foot.