diff ratio, much of a difference?

Mathematically,You might be trapping in second gear at 6400, which while a little high,ain't all that crazy,for that cam. Or in Drive at 4400, choked pretty good.
4.10s would get you down to 6200 now in direct,and 3.91s to 5900, more on target
I'm guessing 2/10s
there can be 5/10s in the 60ft
At 107mph the bulletin book is saying 336hp@3400pounds. And SS ET for that is a tic over 12.4.....................So you're pretty on target already; ET to mph speaking. That's surprising to see.