Hesermon just gave instructions to R&R a small block Chevy oil pump, NOT a turbo Buick oil pump.
The oil pump is integeral to the timing cover, and to really do it right I would pull the timing cover. But......WHY do you NEED to replace the oil pump? If you have low oil pressure, look elsewhere. Time to R&R the engine and check bearings. If it is a rebuilt engine, then it might be a couple of things. A oil gallery plug popped out, bad bearing clearance, spun cam bearing, worn out lifter bores, loose oil pump pick up tube, NO oil pump pick up tube, oil pump clearance way too loose, ect. Still good idea to pull the engine and take a look. Everything needs to be checked, and double checked. I need a little more info on WHY you feel the oil pump needs replacing. I have never seen a worn set of oil pump gears really cause a low pressure issue. I'm sure it could, but after 20+ years of building these things, it is useally a bearing issue that casues oil pressure issues, or a really loose thrust clearance in the pump.