[Mike C]

TH200 4R and a set of 4.10 gears would be a nice setup. Should have you in the low 15's high 14's and be able to cruise anywhere. Built TH200 4R and built 700R4 are about the same $, but I MUCH prefer the gear split on the 200 and it is a bolt in for the TH350 if you just slide the crossmember back. Uses same driveshaft. I have an Extreme Automatic TH200 4R on order to put behind my (factory rated...) 390hp 427 L36 with 2400 rpm converter.

AussieinNC is more realistic with his 750 hp at the crank estimate.

FWIW, it takes about 550 whp to get 4000# into the HIGH 10's. And you are probably putting down 220-240 whp at best. Your idea to just improve what you have is good. But it won't be 10's.