Just as slow as before mods..

The Dana 60 per se is NOT what is eating power, although it certainly will eat some

Refer back to POST2

Tire size?
Cam specs?
Timing?
As mentioned carb opening AND JETTING, what do the plugs look like?

It is pretty easy to overcam a SB with low compression, and pretty easy to overcam a 318, specially with stock ish heads/ valves/ ports