Joe,
I've tried it both ways... premix and OMP. I'm am currently running the pump because of problems I encountered with ensuring adequate oil/fuel mixing in the wing tanks (due to baffles). When premixing, I left the shaft in the front cover and just built a block-off plate. I had no problems with that for over 200 hours. I had heard that there was an oil hole that lubricated the oil pump chain. Not wanting to lose my oil pump, I left things as stock as possible. I was glad it was there when I decided to switch to the OMP system.
Mark