Spray threads down with Liquid Wrench or PB Blaster or favorite recipe a couple few times a day and hammer it hard to shock the fastener, it will wick into threads better. After a day or two or three it should be a lot better. Whatever alternator decision do yourself a favor and get brand new heavy duty ground and starter battery cables of sufficient gauge, to frame, everything, from your friendly tractor supply store. Old corroded cables will cripple any alternator output no matter how many amps, good cables mean you'll get all the juice you paid for.

Well hopefully it won't take that long. But it really helps to give it some time to work and soak through, esp. if it's rusty. Take your time. If it's your daily driver, well that may be a problem. Test out your battery too, get it cleaned up, tray &c, put it on a charger, maybe think about replacing that too if it's getting more than 4 or 5 years on in age. Like I was sayin', get good heavy cables. The generic replacement cables in the blister pacs are crappy, they may work OK for a small 4 or 6 cylinder car but they are not enough for a truck, especially with a plow. Really have to make sure the engine grounds and frame connections are 100%, and the alternator itself & brackets are clean and tight to the block itself with no grease or paint.