Terry - in addition to all the fine advice you will get here, you should run a heater hose over to an inline coolant filter. Your flaps should have them. When you knock the crud loose this will help to remove it - even if you only leave the filter in place temporarily. Otherwise you may do more harm than good. Would have saved me $3K in repairs to my diesel had I had one.
The best stuff we have found for removing rust - to the degree possible on an old truck is a f*rd product called VC-9. Not exactly cheap but it really does a good job.
Good luck
