We run about the same setup, tires, etc here in Texas, a friend of mine tried the 3 wheel brake thing last year and when he would set it up right it would fly around the corner, if he was off a lil bit he would spin himself out. i would say he spun out more than he made it around the corner and if there was a wreck in front of him he was 95% guaranteed to hit them due to he couldnt stop in time. I personally like 4 wheel brake setup with a good Hawk Brake pad in the LF and a cheap on the RF. I use a wildwood master #WIL-260-4894 and no booster with stock drums in the rear and a 10lb wildwood residual valve to keep the pressure up a lil for the rear, i placed it just off the master going to the rear per wildwood tech. It doesnt feel like the old manual brakes and it doesnt feel like the power brakes kinda in between but more close to manual type feel. Im gonna try WIL-260-11179 this year, it takes the place of the combination valve. The master is stock appearing/replacement and they say nothing about a residual valve in our rules. Do a lil reasearch and look for a BB in the brake line and combination valve internal tricks "mud truck guys do this sometimes" that will give you some ideas also. They also make a combination valve specifically for a front disc and rear disc for the metric. what you have on there now is a disc/
brake drums combination valve sounds like. JMO and everyone has got one and it sounds like you raced in the past so do what feels best for you.