[BOBOCobra]

Sweet spot should be no more than 195.
Take it to a rad shop and have them boil it. The heat will loosen up any gunk in there and they can flush it out with pressure washer with hot water. Backflush the block to get the jackets as clean as possible in case there is crap in there and all your doing is pushing crap around.
If you want to try at home disconnect the upper and lower hoses. Take your hose and a 100 psi air hose with a long nozzle and trigger and stuff into the lower outlet and wrap with a rag to get it as water tight as possible, let the water fill up the rad so it is coming out the top inlet and then hold the rag, hose and air nozzle tight and blast with air. it will force a lot of water back through the core and out the top inlet. you then need to let it fill up again with water and repeat. Do it 4 or 5 times. If the water is dirty do it till its clean. It will make a mess but it gets a lot of volume through the core quickly until the air takes over and you have to refill. You can leave the upper hose on the rad and try to direct the flow away from the block. 2 person job, The problem is the water is cold and it wont loosen any hard gunky crap stuck in the tubes but it will flush the tanks and any loose stuff. Or, you can buy a new rad and do your best to backflush the block before you put it in. Ron Davis in Arizona will make one based on your CID and space. He is a good source to talk to.