3 SPD shifter mounting

As to the factory boot
The boot comes in two parts, the lower one screws to the hump,under the carpet, and is the weather seal, keeping out dust and water,etc. The upper boot is the dress-up boot and with the trim ring sits on top of the carpet, and is screwed down thru the carpet and also to the hump, thru holes in the lower seal,provided for that purpose.

Part of HotRodding, is making things fit, and all these years later, there is a lot of that going on.
Happy HotRodding