It is interesting that your Eheim had real problems with that radiator. My 350 GPH Danner was pumping over 1 GPM through mine, even when put through the entire system loop. Check my Phase 2 article dealing with the bypass loop around the radiator before you start that project. You'll get better results if you try to stay at 3/4" net ID through your system with a pump that powerful. You'll get better net flow allowing the blocks to do their job better. Here is the link:
http://users.adelphia.net/~jnsholcom...erproject5.htm
Let us know how your project comes along!