Have a machine shop pressure test the head instead of magnafluxing it. There are places a crack can hide that magnaflux can't find. By pressurizing the head all the hidden spots are tested. It's also possible for cracks to be in the block, not the head, such as around the valve lifter area and the bottom of the water jacket above the crankcase. It's also possible to pressure test a block, but it's a lot more work than testing a head.
You could take the valve cover and side cover off, fill the radiator with water, and put a few pounds of air pressure on the cooling system. Bypass the heater core first, and don't spring any leaks in the radiator by overdoing it on pressure.
Jerry
