Gday Brett,
The only way the radiator places rebuild radiators is to replace the core, and utilise the original top and bottom tanks, or left and right tanks if it is a Cross-flow variety, if they are any good.
The tanks virtually never give a problem, and if you only want your radiator cleaned out, then they remove the tanks and clean out the water passages through the core by both chemically cleaning it, and rodding it. By that, I mean, they simply insert a special rod that dislodges any gunk that will limit or block the flow of water, and physically push it out of the tubes.
The trouble with most radiators is that the actual transfer tubes become brittle after many years of road vibrations, and being bombed by bugs and road grit, that they might still hold pressure, but not for long. And, further, the cooling fins between the transfer tumes are even thinner, and although no water passes through them, they become blocked so no air can get through to cool the whole shebang.
In my many years of build Hot Rods, performing engine transplants, and conversions, etc, I have become a great friend of the local Radiator Shop, and they know just how fussy I am, and that when I come into the place, they have to do another unusual job for me.
The main thing with increasing the cooling capacity is to get the largest core that will fit the tanks, and if necessary, increase the size of the tanks.
Once I took the radiator support of a 37 Chev Coupe that I was building and said, Here is the place where it has to go, there are the mounting brackets and there is where the filler, inlet and outlet pipe (and size of each), has to go, and I want to cool a street driven 600hp 427 BBC and run thermo fans. Well, we sat down and devised a plan and ended up with a beautifuly fitted 3 core radiator that held 15lbs pressure, and never once looked like overheating, even when towing a caravan in 104 degrees F crawling traffic. The original radiator actually worked well, but it wasnt a pressure variety.
As far as the Heater core goes, it would be more protected from road grime than the radiator, but I would still be getting it checked. Especially the water control valve.
Hope this helps,
Bruce,
The Tassie Devil(le),
60 CDV