The thing to keep in mind is to call TCI and find out how much it will cost to change the stall on this used converter, and add that what you are paying for the use converter. I'm sure you don't want that total figure to be more than what a brand new converter will cost.
The ball park stall number you should be looking for is 1,000 to 1,500 RPM lower than your shift point. You see if you have a converter that stalls/ flashes at say 5,000, but you motor "comes on" at 6,000, your combo will struggle for 1,000 rpm. BTW converter flashing is where the RPM drops to right after the shift, which is the true stall speed. Plus if you get your converter working where you want, and then you increase your power, then the converter will flash at a higher RPM.
Good Luck, and have Fun
Jon Hansen, JrFuel "B" , Hayden Wheels, and JrFuel Dragster Association