The diff plug on my 84 Berlinetta is stripped out, the square hole it's all rounded out. I've been trying to find a new plug to order but nothing seems to "fit my vehicle." So I was wondering if anyone knows of a good replacement plug and how to get the old one out.

 

Pretty sure it's ¼" pipe. You can pick up a new one at Home Depot, Lowe's, Menards, etc. Piece o cake. No need to "order".

Getting the old one out is a bit ... tougher. The factory plug is steel, and it screws into the cast iron that the pumpkin is made out of, so over time they tend to become ... one. Sometime all you can do is drill out the old one, being careful not to damage the threads in the pumpkin, and pick out the fragments. Slather yer drill bit with the thickest nastiest gooiest greeeeeeze you've got, and magnetize yer drill bit (rub it across the strongest magnet you can find, like an old bass speaker or the like) so that the metal chips stay stuck to the bit. Butt then, put a BRASS plug back in, so it doesn't rust itself into oneness with the casting like the "genuine GM" factory s*** did, so you can deal with it in the future.

 

IIRC, the square drive is 3/8". Try tapping slightly larger square stock in and twisting. Slot it and use a chisel or punch to get it to twist next. Welding a nut or the square stock to it would be a success since that heats things and gives a new drive surface. Alternately, drill it, tap for LH thread, and use a LH thread bolt head to spin it out.
A new cover with drain and fill plugs is an option. The stock plug is a std. NPT size.