Thanks for all the helpful replies. I tried the easiest method first and it worked great, see before and after photos below. I let the radiator cool then used a brush attachment on the shop vac to get the easy stuff off first. Next I used a nylon bristle brush I got at the home center to carefully brush off the stubborn stuff by going back and forth and up and down. I then used a crevis tool on the shop vac to clean up the debris that had fallen off the radiator and collected at the bottom. Finally I used a small pin to straighten out a few of the fins that got bent by bigger bugs. Sweet.
Before:
After:
Tools used (except the pin):
