You probably won't find that combination in a Chilton's manual for the Regal. The Buick Regal that I had years ago was originally a V6 that I installed a V8 into it. But get a manual and look at some of the other vehicles that had the 350 to find the Removal and Replacement methods necessary for the V-8.
The oil fill would be to pop one of the breathers and put it in through the valve cover. Only the older 262? 283 327 had a separate oil fill provision tube in the intake.
Correct on it most likely being the rear main seal needing replacement, but make sure that PCV valve isn't stuck and that the breathers in the valve covers are not stopped up. These problems can cause the crank pressure to elevate and push oil out the rear main seal.
If you do have to pull the pan, you do have to unbolt the motor mounts and just about pull the motor out to get the clearance.