What kind of performance are you going to be looking for? What are your minimum acceptable range & speed goals?
My suggestion: think about doing a 72 volt conversion. That will permit you to use a series/parallel setup with the 36v forklift controller for smooth, lower speeds @ 36v (2 strings of 36v in parallel mode), and switch to 72v (2 strings in series) for speed.
It'll also let you use your 36v charger - you can charge both strings in the pack in parallel mode.
Very exciting.
