Look into a Flow-Kooler water pump. They move the water much faster than stock at low RPM then revert to normal flow above 2500-3000.

http://www.flowkooler.com/

I have one on the 350 in my Studebaker and it works great. Also add a shroud around your fan if you don't already have one.

Also, I had a 63 Nova with a small block 400 and I don't recall radiator to waterpump clearance being a problem. Are you using the short waterpump? You can also mount the radiator in front of the core support to gain clearance if you have to.

Hope this helps

Pat