I am wondering, in advance, about your dilema as I experienced it on a earlier-model 4R70W with the J-Mod. You haven't tried the stock springs in combination with a larger (0.109") 1-2 restrictor plate hole?
I am wondering if this would provide the right combination of firm-without-slamming shifts.
Also, how does your computer sense load? Do you have a different MAP sensor (and corresponding tune) that measures boost?
Lifespeed
2004 Silver Birch Marauder 130K miles
Wilwood Aero6 F, NMDP R brakes, Penske 7500DA shocks, Hypercoil 600lbs F and Grand Marquis R air springs, Addco tubular sway bars, Metco control & Watts, 31 sp axles, Stainless Works cat-back exhaust, Lidio tune, American Racing AR883 9.3" 50mm F 10" 59mm R 20" wheels, Pirelli P-Zero 265/35-20, 275/40-20 tires
Coming soon: Livernois 5.0L stroker, ADTR/Vortech V2 intercooled SC, Silverfox trans and Circle D converter