Being a lockup, you're correct, when it's locked at speed the stall doesn't matter. The difference you may notice is the converter lag off the line more with the cheaper converter than a custom one. more constant rev, less flash... I guess the question you have to ask is what is the goal for the car, and what is your current engine/combo set up for... I'm guessing by the fact that you have a 3500 on it now, you might need a bit more than a 2600... It's better to spend the big $ once than to spend $ twice and pull the trans again...
Joe