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If residue from antifreeze, oozes out while you solder, it will be a problem. It's best if the hole can be positioned straight up, but as you said, you want to avoid removal. If you have access to the hole, I think you can do it with the radiator in place. At any point in the solder...

I use a nice big electric iron. Lightly sand it or use Scotchbrite, to clean the immediate edges of the hole. Flux it and the clean iron(I actually use a file to clean my iron tip). Tin your tip and touch the tinned tip to the hole, feed some solder on and pull away and see what you have...