Welcome to CHR Otis:

I think that you should seriously think about replacing the transmission. With 200K miles on it there’s bound to be some wear and fatigue. Stock 350 in the ’85 trucks put out 175-210 HP with 275-300 lb-ft of torque. A conservative 383 roller build will be in the neighborhood of 350-400 HP and 425+ lb-ft of torque. The stock 700R4 is good for 300 HP and 350 lb-ft of torque when new and tight. Unless you run some very skinny tires that will spin forever the first time you nail the pedal and the tires lock up and you bang second or third you’ll split the converter, dump the fluid all over the street, and be hearing some real bad noises.

Heavy duty rebuild on a 700R4 is good for well beyond the horsepower and torque a 383 will deliver in all but extreme race configurations.

My two cents.
Glenn