They are actually fairly durable. They just need regular fluid changes. And don't use the transmission to stop it changing directions, use the brakes.
Put a good torque converter in it. The lock-up on the stockers is a bit weak, then they slip and put debris through the rest of the system. It's a cellulose(I.E, paper)-based friction material. Do some looking around online, I don't know if they build dual or triple-disk convertors for the gas engines, but there are better ones than the stock ones.