As a Gene-esque aside, The radiator in my 73 was always oozing smegma from various solder joints so while I had the engine out this year I took it to the only radiator repair shop left in the entire Front Range of Colorado, which happens to be in the city where I work.
Dirty, cluttered, dark, sloppy, messy little place run by two guys just like that too. I was very not impressed but they are the only place left who rebuilds radiators & heater cores rather than just sending out for aftermarket replacements which don't usually fit. And they have a very good longstanding reputation for knowing their shit. (I've done safety inspections there and could write them up on so much stuff they'd be out of business tomorrow, but sometimes I try to look at a bigger picture, so I overlook a lot and only ask them to fix a couple things at a time. And I will do the same again next time and next time.)
So I took my OEM radiator in for them to repair, and when I went back to get it they showed me the parts, explained that it was too far worn to repair, the side tanks eroded too thin to use again. Then he pointed to a tag soldered on the side tank and said, "We repaired this very radiator in 1978. That's the tag we put on every one we work on. It lasted a good long time, don't you think?"
I had to smile and agree and ask "Well, what now? can you help me?" He said "Yeah, but all we can do is order you a new one which probably won't be exactly the same but you should be able to make it fit." (He knew the problems associated with fitment of aftermarket 'Vette parts, we had talked about it on my inspections....) He didn't charge me anything since he couldn't fix mine, even though he had time into cleaning & disassembling & inspecting it.
So I told him,"OK, I'll trust you to know best. Order me a new radiator. A good copper core equivalent one."
When it came in a week later, the bill was $400. I noticed a shipping sticker which was to the local Chevy dealer. So a couple days later I stopped in there and asked them what a replacement OEM radiator went for and the parts man looked a while and then told me, "Well, it'll take us a while to locate one, the last one any of the regional warehouses had got shipped out last week. It'll run around $600 or so."
I told him my story and he chuckled and said, "You got a deal, our cost is about $375. Yep, looks like it came through us and we charged them no mark-up." (I've done inspections at the Chevy dealer too, and BS'd about old 'vettes, so the parts man recognized me and was at ease telling me what the real scoop was.)
No, it didn't fit quite exactly like the original, the side tanks were larger which put a pinch on the lower trans cooler line to the frame, but I got that fixed with a grinder to the frame horn and a $20 bill to a local hose & fitting shop to tweek the bend on the metal line.
That same Chevy dealer--OK, plug for them, Ghent Chevrolet, Greeley, CO--also found me an OEM GM heater core a few years ago. I specifically asked for OEM not aftermarket. Took them around two months to find one, he said it was the last one out of a Canadian warehouse, and it fit.