The rust definitely looks quite bad, I lowballed hard when buying this car because of that (and the visible delamination).
I had a few of the ex-midwest and GM production guys from work take a look with the fancy borescope, and we're all pretty confident that the chassis doesn't need to be pulled, but another 5 years of improper storage and it would. I took at look at other removed frames and none of the common areas have anything significant on this one. Originally imported to Texas, then moved to New Mexico around 73 so that probably helped.
The front face pictured there is by far the worst area (makes sense, any rain would blow directly onto it), and around the master cylinder is a bit weak, so it's getting a large doubler plate that also covers the nasty steering column hole. New MC from R.Dent barely doesn't fit the hole so that'll be opened up slightly too.
Anyway, just stopped by WestMarine and picked up more epoxy and thickener to hit those vertical surfaces. The hesitation to over-restore is very real, but somebody already glassed a S2 front end onto this thing so I think I'm in the clear
I didn't think about doing cloth and then mat over it, great idea. I was going to leave the interior largely unfinished for a while. A lot of the mat stuff isn't meant for epoxy, as only polyesther can dissolve the weak binder, but apparently it's still mostly okay? Nice article here:
https://www.epoxyworks.com/index.php/chopped-strand-mat-and-epoxy/