[mr0077]

Elmariachi, it is either flow or radiator, but you're right, from what you've said, I'm betting it is a radiator issue. If the size is/was okay, I'd bet it's plugged with crud. Use an infrared temp gage to check various places on the radiator. If it is flowing properly the temp on the core will be fairly uniform all over the radiator. If you find cooler places, take the radiator out and flush it to make sure there's not debris in it. If nothing flushes out, have it professionally cleaned.
Also, small things can make big differences. On my first Cobra (a big block 460) I had a similar problem, and solved it by sealing the fans very closely to the radiator...my overheating problem at idle was caused by air loss around the shroud, and I only had a 1/4" gap!
BTW, 70% water and 30% coolant is a fine mixture, that is not your problem. If it was a faulty pump it would overheat worse at speed. Advanced timing isn't the problem, although you ought to have yours all in by maybe 2500 instead of 2000 rpm. Retarded timing can cause overheating, but yours sounds fine.
Keep us posted, someone here will help to figure it out.