Well here's the report:
The heater that was in the truck was a Studebaker heater.. as reported in my original post, it looks like some kind of toaster oven.
The bolts and pipes didn't match up with a Chevrolet heater, so whoever installed it had to drill their own holes.
Apparently this truck didn't come from the factory with a heater, because the "knockouts" for the factory heater holes were still in.
So with the old Studebaker heater out, I tried to knock out the factory knockouts.. which didn't work so well.. I ended up using a 1/8 inch metal bit to drill a pilot hole in the center, then used a stepper bit to hog the hole the rest of the way out.
The AD heater stud bolts line up perfectly with the factory knock out holes.. the plumbing (water inlet/water outlet) holes are about a 1/4 inch off, so I just made both holes a little bigger and she slid in nicely.
I'll have to rig some duct work up to the defroster vents in the dash, and silicone the old Studebaker heater holes, but the short of it is, the installation of the unit goes pretty smoothly, and looks like it outta work.. and as a plus, it looks like it belongs in there.
