Auxiliary transmission cooler

I'm about to swap in a shift-kitted 904 with a 2800-stall converter into my '70 Duster and I think I should get a transmission cooler for it, since I'm planning on driving up to the mountains and thrashing it pretty hard. What ones do you guys recommend? I'm looking to spend less if I can and I'm willing to do a little work getting a 'universal' one to fit. Mainly it's the size I'm unsure of.