What is needed to build a 7 second (1/8th) street car

car is a 71 street/strip Duster already has a built 727 with a 3800 stall. What kind of HP and gears are needed to push it into the 7-7.40 range on an 1/8th mile track? It is turning an 8.20 now with a stout roller 340 and 3.91s car is set up for a 150 shot of NOS but id rather not use it.