I have one on the right hand side also and had to angle it somewhat in towards the car to get it so I could see out of it properly. I took the arm out of the base and fettled it a bit to get it to lower a bit more, but that wasn't really necessary. The main problem is angling it in towards the car (I wanted to use the existing holes in the door and not drill new ones).
I briefly considered taking the mirror apart to see if I could get some more movement of the glass, but didn't know how it was arranged inside and if I could get it back together - but looking at Johnny's post I think that is definitely the way forward - take it apart and carefully file the slot a bit bigger and then reassemble it. That is what I will be doing shortly! The only possible negative with that is from an appearance perspective you may not like it if the glass angles out past the edge of the mirror housing.
Apart from that, I think you should definitely test mount the mirror. I know of at least one Pantera owner (not me) that drilled the holes too far forward on the door on the right hand side - the frame of the quater window is in the middle of the mirror's image which is somewhat anoying for him. Sit in the car in the normal driving position with hands on the wheel and back on the seat back and get someone to move the mirror around in different spots and with different angles to make sure you can see properly and then get out and see how it is positioned and if you like it or not - and if not then decide how to either modify the mirror or make a spacer plate etc. This may save some tears
Good luck!
Cheers, Tim.