Try throwing a 9.5 or 10.5 power valve in it to see if the 4.5 is just too late. The low speed is plenty fat. At what RPM does the cam smooth out? It is hard to get the low speed right while there is still cam lope. Without getting into another big debate as to how to do it, I would size the IFRs and the IABs both down about the same percentage and see if that affects the low speed, how much and to what RPM.

You may also need to address the slot, but try those 2 things first. Then go from the results.

And A/F ratios at RPMs rather than MPH would probably be a bit better if possible. Are the A/F ratios and MPHs given all in one gear or was the tranny shifting?