A "drift" is a rod you use to focus a hammer blow on a small area. A "brass drift" is simply a drift made from brass (a "drift" differs from a "punch" in that a punch is build heavier, and meant to drive pins that fit really well.) I've got some 3/16" brass rod that I use for RMS drifts.
Why brass? Because it won't score the crankshaft if you slip or miss. I also suggest using a brass, wood, rawhide, or plastic mallet in case you slip with that as well. The transmission doesn't have to come out to change a rear main, it damn sure does if you have to swap a crank!
I've done the job twice on stockers without having to remove any of the front suspension - just jack it up, support by the frame rails, and let the front axle hit "full droop." It's easier with lifted rigs, but you can get enough room without. The starter does not need to come out, but it does help.
Use the 1996-up oil sump gasket - it's one piece, and saves you trouble. I think the RMS is the same all the way up (two-piece rubber over a metal core.) Drop the new seal in clean engine oil before you start working, and let it soak. This will make it easier to install.
If you suspect multiple leaks, don't neglect the O-rings in the oil filter adapter - especially since they can present as RMS leaks until you inspect closely (a blown rear main will show radial oil streaks on the engine side of the flywheel or flexplate, while an oil filter adapter leak or a valve cover leak will not.)
If the oil sump hasn't been off yet, expect to have to bang on the pan to remove it.
You can get little "stripper discs" that look like 2" wheels of Scotch-Brite to mount on your drill motor - these will not only strip old gasket & sealant quickly, but will give you a better finish for sealing the new gasket as well. Check the paint section or tool section at your local.
Oil sump screws - 1/4" screws torque to 7 pound-feet, 5/16" screws to 11 pound-feet. DO NOT OVERTIGHTEN!