I don't know the weight.
I was thinking tranny or converter slippage also.
Or the tach is wacked and not reading correctly.
The converter is an ATI tight.
I should send it out and have it checked over.
I would have the converter checked also especially if it hasnt been in a while or you just dont know its history. Good converter people will get the whole combo from you including the weight before wroking on it so take the car to a scrap yard or moving company and get a general weight on their scales. Try several tachs if you but just get a good one from a buddy on a known car for reliability.
340srule suggested a 4.30 gear but I'm not sure that was one or your original choices. A 4.56 will work with a 28 inch tall tire and 6500 is still safe for a well put together bottom end even with the TRW's.
Based off a pretty reliable calculation with a 4.56 gear and 28 inch tall tire to to attain 120mph which would genarally put you in the low 11's high 10's 6500 would be the approximate trap rpm.