Well it looks like you have quite a good combo there. You should be running 105+ mph. Your 99 is way low. First things to address is how is your fuel pump/injectors etc. Do you have an adjustable FPR? What is your timing at? Since this was a recent install is everything correct, have you done a compression test etc?
I still have a 2 1/4 cat back exhaust system. So that is not a big restirction in your system. There is something else holding you back. Is your MAF calibrated for the injectors you have? Yes more gear will help you. But the gear or the converter are not the problems in your current combo.
We'll help you work it out don't worry.
Skyler
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-1989 Saleen Mustang #406- TFS Heads, E-303, edelbrock intake,70MM TB, 73mm MAF, off road H, headers and 3chamber flows.
12.55@107mph
50-175 Nitrous Works WET Kit just added.