Here's my train of thought on the idea, and maybe this would have been helpful to include in the OP.
First off, the car will be accelerating between 20 and 50-55 mph through various parts of the course. In order to go fast, I'll have to keep the revs up at those speeds at a level where the car doesn't fall on its face when I pick up the gas coming out of a corner.
We happen to know that Low gear in the 4R70W is 2.84:1 and Second is 1.55:1 What I'm not familiar with is what the NPI 4.6 likes in the way of revs that keep it in the meat of its torque curve.
The trans cooler goes without saying. I have a hookup on nice trans coolers. I will need to find a shop in my area (Evansville, IN) that I can trust to do the J-mod. The converter will need changed because I have a feeling that in order to keep the revs where they need to be I'll be ballooning the stock converter.
I found a neat tool online for calculating RPM based on tire size, speed, trans ratios, and third member ratios: http://hotrodstohell.net/rpm_tool/rpm.htm
According to it, in order for Second to be useful from 30-60 (2700-5500 RPM) I'd need a 4.56 gear. If I have to brake to 20 (which I will) the revs get killed down to 1800. Pushrod engines aren't especially effective at that speed so I doubt the SOHC motor will like it, either.
If I leave it in First, picking up the gas at 20 and accelerating to 50 has me revving from 2200 RPM to 5600 with the stock 3.08 gear. Maybe that is an option?
I don't intend to run anything lower than 4.30 as this car will not be towed and will see regular civilian street use.