I'm not familiar with the Drifters, but I always make sure the Invader oil pumps don't leak before installing it. The motor needs to be removed from the sled to change the pump, nothing worse than putting a sled together just to find out you need to pull the motor again. The Invader pumps like to leak around the shaft. They don't leak much, but it can be a nuisance. Cleaning the pump thoroughly, then filling it with oil and let sit on the bench overnight will tell if it's a leaker.
Good luck!