I've been holding off buying a replacement heater core to replace my original, factory copper unit. My OE one isn't leaking, but it's also 30 years old and since I have everything apart, it would be stupid not to replace it.
However -- all of the aftermarket replacements are aluminum "universal fit" models - nobody makes a proper, exact fit replacement (aluminum of otherwise).
So tonight I was checking eBay for NOS GM parts and much to my surprise - someone actually had NOS GM heater cores (GM 3048945) for only $58. Needless to say, I snapped one up and now have a correct, genuine copper heater core. I still can't believe it. I'll keep my original just in case, but I figure the first one lasted 30 years without leaking and if this new one does as well, I'll be money ahead.
I also picked up some GM spark plug wire supports/holders. Again, I wasn't expecting to ever find these. My original holders, like most of the plastic under the hood, were cooked and brittle. They completely fell apart. The GM F-Body Parts Catalog shows the part but does not list a number, but these look correct. If they are, it's another small eBay miracle.