I believe those kits are just the turbo coupe rear disc conversion with 4 lug discs.
Problem with that kit is they use the 45mm rear calipers and you basically have to use that with the upgraded Lincoln 73mm front calipers to keep brake bias in check. If you use that kit with stock front calipers you will have way to much rear brake bias....Yes, even with a proprtioning valve.
But that setup is factory stock on the 1993 Cobra using the 60mm front fox calipers and 45mm rears. I've seen many copy that exact setup without much complain as to too much rear bias. The base model 92-93 Saleen setup did use the 73mm front calipers, with 45mm rear calipers stolen from the 91-92 Lincoln Mark 7.
I can't imagine a 10.5" rotor and 45mm caliper is generating more rear brake torque than an 11.65" rotor and 38mm caliper. Can't ignore the mechanical advantage of 1" greater rotor diameter
Anyway, reading into the OP's question, it sounds like he's looking for a way to add rear disk brakes, but keep his recently purchased 4-lug 31 spline axles. Yes, there is a way to do it, but he'll need to purchase additional parts on top of the two kits he mentioned, which are basically 93 cobra/87-88 t-bird rear disk setups.
North Racecars Brake Solutions
Basically he'll need to spring for the $170 to buy a different set of brackets to move the rear calipers inboard 3/4" and run his current 31 spline axles. This will also allow him to run wider rear wheels than he could if ran the setup as is and pushed it out 3/4" each side.
There is also a way to modify the existing brackets by cutting them and mounting them inboard of the axle flange.
Of course, then he'll need MC, booster, and prop valve mods, not to mention adapting the parking brake