Thinking about replacing tranny fluid and filter.

Well my blazer just hit 120k miles, and im thinking its about time to do a tranny fluid replace and a filter. I dont know if its ever been changed judging by the factory spark plugs still being in there its not very promising. So i know i need the filter, fluid, and a pan gasket. Ive already had one tranny decide to quit on me, due to unknown reason till i pop it apart (from another truck). Just some questions how much fluid does a 89 4x4 4.3 need, its a 700r4 trans. And also ive heard that replacing fluid after a certain amount of miles can hurt the tranny. Im pretty sure its a myth, but just double checking i dont need another trashed trans. Also is there anything else i need for the oil replacement.

Should take about 4.5 to 5 quarts. The exhaust crossover will give you fits: you'll have to loosen the the "Y" pipe at the manifolds. With 120k, as long as you don't have the transmission flushed, it should be ok.