Which gasket should I use when I have a 5.9 Magnum engine while using a LA style 360 oil pan? I like how the magnum style gasket is a 1 piece molded style, but is it worth the extra $15? And if I go the LA style would cork or rubber be better?

I also need to replace the rear main seal on the magnum engine. Whats the best route to go with on this?

you have to use the la gasket.
if you try to use the magnum, it leaves a gap and oil will just pour out as you tilt the engine to get it in the car.
Or you could goop the crap out of it, or bend the pan.

I use the magnum pan gasket on all 360LA engines. Never a leak and reusable. Yes,it takes a few minutes to correct the 2 pan corner notches,but its so very simple that a caveman can do it. Bending the pan or gooping it up are not required.

I already had the magnum gasket for use on a magnum block with LA style pan. I trimmed up the rear to thin the cap portion. Right Stuff in all 4 corners and good to go. If they had a 1 piece rubber version of the LA gasket, I'd use that. Not sure if it's available though.

After looking at some old posts about the magnum gasket on the la pan, it seemed like it was a toss up on whether it would work or not and since I really don't want to have to do this twice I just went with the LA style 'crush proof' gasket from milodon. Hopefully this will work okay.

For the rear main seal I just went with a fel pro stock replacement. When I'm changing this, I pull off the rear main cap and push the old one out and slide the new one in correct? and a little RTV at the corners?

Some rtv film between cap and block also.