radiator. what should i shop for?

Aluminum radiator will cool MUCH better (more efficiantly) than a copper/brass unit of the same size. Aluminum has better heat transfer properties, and most alum radiators have much larger tubes than copper/brass units.
You are limited to 22" wide in an A Body due to the size of the opening in the rad support unless you plan to do some cutting.
I have a Bee Cool cross flow alum rad in my 69 Dart. It's 22" wide and 19" tall, and with a flex fan & plastic /6 shroud (installed upside down) it keeps my built 440 COLD. If I don't run a thermostat the needle on my temp gauge barely comes up off the stop pin.
These radiators can be had for about $300. Might be a bit pricey at first, but if you buy something inexpensive, it might not do the job, or might not last very long.

If you want some pix, just ask!

Good Luck!

George