The modulator screw is right beind the vacuum hose, just remove the hose and take a small screwdriver to it. There is no need to remove the whole modulator. It would affect both shift points. If it is shifting that early, you might want to try turning it in 2-3 turns. It think it would be better to go too much and turn it back out then having to go 1/4 turn then drive and 1/4 turn and drive etc. There is another thing but is alittle more indept. The governor weights and springs could be at fault. Since it the transmission was out of a heavy car the governor is for a heavy car. A diferent set of springs and weights could make the difference but that is more of trail and error. One final thing, what gear ratio do you have in the rear end? The higher the ratio number, the quicker the shit may be. Hope this helps.