When you get parts from any catalog, they never really have good instructions!   I am replacing after market side mirrors with orginals on my 69.   I dont know if old holes are still there, or if the even was a stock right side mirror???  The kit I got has "well nuts" that need to be installed in the door skin.   Do they just tap in???   No glue or anything to help hold in????  seems like it would very week, being the fiberglass cant be very thick??? 

Your car would NOT have had a pass side mirror from the factory, but they could be dealer installed.
The dealer mirrors have backing plates/nut-serts that fit on the inside of the door, to help strengthen the area. "Well nuts" are similar to sheet-rock inserts, in that they expand on the back side when you tighten the bolt/screws. Without actually seeing what you have, and the instructions, it's hard to say if it is strong enough, but I'd have to think it'd be okay.
The door skins are thicker, and stronger than you might think.

Joel is absolutely right about the fiberglass being strong enough.  In fact, my research indicates they only used the backing plates through '67, and used the rubber well-nuts as the correct dealer installed mount starting in '68.

Larry