Honestly, I'd just assume that the ABS valve is bad on them all. Unless you feel that the braking technology of the 1990s is capable of stopping your truck any better than you, I'd just go ahead and ditch it whether you think it's the problem or not. (if it's still there, it probably is.)
I'm actually in the same boat as you, Scott. I did the 1-ton shoe conversion, new calipers and pads on the front, 2nd new master, (just assumed the master had to be bad, since nothing else showed itself after a few nights of bleeding.) ABS valve is already deleted on my truck, and it still has a random spongy pedal. I had a nice pants-filling moment when I decided that yellow light wasn't going to last quite long enough, and ended up dragging 1 back tire through the red light.
My vacuum leaves something to be desired, as it seems to randomly work, and I can hear it leaking out the booster so that's kind of my fault, but I still get the mile-long pedal drop at times. My MCs also seem to leak out of the lid. The stock on did, and so has every one I've gotten so far. I just space the gasket out from the lid with some thin rubber.
So, delete the ABS, (it'll be a little in-line sensor on the drivers side above the rear axle) ensure you've bled everything, make sure your vacuum pump is functional, and that your booster is working as well, and make sure your master isn't leaking out it's fluid through the lid.
EDIT: And tighten up those shoes, as suggested by Mr. Nixon ^^