I ended up pulling my coil harness I made completely out of the car last night to go through it and make sure I didn't screw something up. Surprisingly, I didn't wire anything there wrong. So I plugged it back in and went through each of the 8 plugs with a volt meter... All 8 of the signal ground wires back to the computer had a rock solid 5 volts.... Uhhh wut?!
Turns out my 1990 mustang has a 1991+ engine harness in it.
I soldered the SG to the correct grey/red wire, but then ran out of time. I have a few more checks and test to run, but I can't think of anything else that could be wrong, short of having a set of fried coils.....
Famous last words, right?