Need heater box & advice - '81 K10 p/u w 350 & a/c

[Frankenstien]

Hey all ... I was hoping someone could offer me a little advice here ...

My newbie attempt at a heater core replacement job has gone awry ...

I was on the verge of getting the last heater box (through-bolt) nut completely off when it jammed up tight
(due to 31 years-worth of crud, and, my failure to be patient and first clean the exposed threads --> lower left through-bolt, viewed from engine-bay side of f/w)
and the unique-looking bolt-head/clip
(interference-fit? / stamped? together right on the heater box, or, are the bolt shanks square and fit in square holes molded into the box itself? --> lower right, viewed from interior side of f/w)
let go and started spinning internally.

My next trick was to set up a small hydraulic jack (inside the truck interior, in the passenger footwell) against the said bolt-head/clip in an attempt to hold it while I tried to work the backed-off (but jammed) nut back and forth on the stud ... after a liberal dousing of WD40 this time.

This did not work, and, the lower right (viewed from interior side of f/w) corner of the heater box broke and has nearly completely broken away from the rest of the heater box.

I will try to get a pair of vice grips on the exposed portion of said stud (between the nut and f/w) on the engine-bay side and see if I can hold it that way. The fact that I have a/c and its' proximity to the R inner fender makes getting my fairly large hands in there quite difficult.

By whatever means I finally get it apart, a few Q's occur to me.

Is the kind of box damage I have described repairable?

If so, what is the best alternative to the stock (unique GM design) bolt arrangement that was used? And, have I got them all? ... two on lower edge of box, one on upper edge and a large hex-head screw on the far R (interior side). I am also assuming I don't have to disturb the a/c shrouding on the engine-bay side.

If box is not repairable, I don't yet see any after-market heater boxes available new online with the usual vendors (LMC, Classic, RockAuto) ... my search here on the forum turned up 23 threads' worth of reading for 'heater box' (which I will get to shortly) ... meanwhile ... can anybody say whether these heater boxes are generally available here on the forum?

And, are there heater box part variants for trucks with or without a/c, having different motors, or, any model years within the '73-'87 model year range?

As always, any help offered will be greatly appreciated ... thanks