If you're worried about making a strong weld, V the parts that you're joining so you have a 'gap' that you can fill. In this case you'd be starting in the middle(center) and working your way to the outter edge. Or at the very least leave yourself about a 1/8" gap between the two pieces so you get better penetration. The down side is things tend to warp more. Only reason I mentioned it is your picture of your tacks, it looks like the pieces are butted right against each other. You shouldn't have any problems with that extra bracing you did, well done and it looks good too. All that's left is paint.

I've welded in a lot of nuts and haven't had any problems *YET*. Does anybody know how much of the nuts tensile strength you lose when you weld it in?

Your little tabs are cool, and should do the job of holding the nuts so you can get the bolts out, but can you get the nuts back into position if need be without removing the radiator? I don't suppose you have room or the inclination to "cage" those nuts so you don't have to weld on them and they won't go anywhere either?