I would guess that you should be running about 5.90's, as I built a 388 for a guy several years ago for a 2350lb car that had similar cam, heads, intake, and a 1050 dominator. It ran 9.20's in the quarter with a 4.88 gear and 32x14x15 and a 5500 stall.
Looking at the rear gear, tire size, and being in high gear, it shows that you should only be up to around 5000rpms to run 103mph. Something doesn't seem right with your ignition box, or you have lots of slippage in your transmission or even maybe the transmission is a reverse pattern shift and you are taking out in high gear and then shifting to low(I did this one on a T400). For you to run 7.50's with the RPM limit set to 7500 with a possible miss, and then run 6.60's with the rev limit turned off either means slippage or an ignition problem.
My car has a big block, but it only goes through at 6150 rpms and that is with 4.56 gears, 33x16x15, and a powerglide that I shift at 7400 and it runs 5.60's at 120mph.
Bill Mitchell