If you looking for the distance just ask one of the guys that have one. Or just sit in the drivers seat to see where you like it. For this you will need a magnet or person to hold it.
I have never seen that base piece on any other 64-66 Imperial. But thats the way it came of this 64 Coupe. I dont know why its like that. All other left and right mirrors just have plastic piece.
Use a stainless wire wheel on a 4.5 inch angle grinder. Then clean it with solvent . Finally paint it with primer. This will solve the problem permanently. Coatings and gels are for the gullible .