1993 Chevy S-10 Engine Coolant

I recently purchased a S-10 pickup truck. I have found that the radiator is realitively new along with a new thermostat and waterpump. (Cracked or warped head, headgasket blown from overheating?) I noticed that I was loosing a small amount of coolant and was gettting a small amount of steam from the tailpipe appearing to be from burning antifreeze. I pullled the spark plugs and found that one of the plugs appears to be "wet" while the others are normal. If I modify the radiator cap not to pressurize, will this slow coolant from entering the cylinder and prolong the engine life? With the high milage, coolant problem and also needing a rear main seal would replacing or repairing the engine be the best way to go?