3:23, 3:36, 3:55 ratios would give you the kind of performance you desire. When picking your cam go with a cam that will work well in conjuction with the gear ratio you plan to use. You will feel a great difference between the 2:79's & a 3:55 ratios. The best 4 speed to use is an M-20 wide ratio box.
I would do as you are, as you can afford upgrades, do them. Engine first, tranny & rear next. There are aftermarket posi units that can convert your rear to a posi.
