Back brakes doing most of the stopping

I have no way of knowing what other modifications were done, but anything that upsets the front-to-rear brake balance has to be considered. For sure a lawyer or insurance investigator will look for these things if their client causes a crash with you, so they can shift some of the blame. They will convince a jury that you were partly at fault when their client ran the red light because you were less able to avoid the crash, and they will be right.

First of all, did you replace the master cylinder? The original one had a residual check valve for the rear drum brakes. That holds about ten pounds of pressure on the wheel cylinders. With disc brakes, that pressure will keep the brakes applied slightly at all times. It also takes some pedal travel to move the rear shoes out to the drums. There's a hold-off valve in the combination valve to delay the application of the front disc brakes until the rear shoes contact the drums. Now that there's disc brakes on the rear, they will apply as soon as you move the brake pedal, but the front ones will not apply right away.

Next you have to consider the added weight in the front. Any GM engine is about 100 pounds heavier than the comparable Chrysler engine, plus you've gone to a much larger engine. You're going to have to push harder on the brake pedal to stop the vehicle, and relatively speaking, the rear is now much lighter by percentage of total weight. That will make the rear brakes lock up much easier.

The engineers spend a real lot of time designing a brake system, and they're different for every combination of factory-optional equipment. The combination valve will be different for a vehicle with a heavier engine, added weight from air conditioning and other options, and the location of that weight. Even a larger radiator can call for a different combination valve. As a brake system specialist, all I can suggest is trial and error to find a combination valve that gives you satisfactory performance. You won't be able to select one at the dealership because they won't have listings for your options. The best I can suggest is to find some from vehicles with rear disc brakes in a salvage yard that you can try.

Most pickup trucks and minivans have height-sensing proportioning valves because those can have such a wide variety of front-to-rear loading between empty and fully-loaded. I don't recall seeing those on Jeeps, but if you do have one, sagged ride height will make that valve pass more brake fluid to the rear brakes. When the rear drum brakes on the older Caravans started locking up easily on light pedal application and wet roads, it only takes 1/16" change in the adjustment to cure that.