1990 GMC C1500 Changing Auto Transmission Fluid

I recently purchased a 90 GMC p/u with 180000 miles for $500.00. 5.7 l V8, auto trans and was bearly running. I installed a new TBI from Holley, new fuel pump, new fuel filter, new spark plugs, wires cap and rotor. Changed map, egr sensors and hoses which all tested bad. The brakes and shocks are need of changing very soon and so is the steering shaft. i crawled under the truck, dropped the transmission pan and chagned the oil and filter. The oil was black as could be. Used 5 quarts of new Dex/Merc fluid. Truck seems to shift better. My question is, The trans oil is still a bit dirty, can I drive the truck for two weeks and change the oil again, repeat this process for two months changing the oil and filter each time until the trans oil seems clean and the transmission is shifting to my satisfaction? I know it sounds expensive but the 700R4 transmission seems to be in good shape but was just neglected. On the next transmission oil change I will install a drain plug to make the job a lot easier. Thanks