On a Borg Warner T96 3 speed with a side shift can I use a 4 speed after market floor shift by using only part of it?
It's do able. Just locate and fix into position reverse arm of shifter so you can't get shifter into reverse position (as a four speed).
Not so much disable it, but it is. It's so the plunger will only be able to work 1 2 lever and 3 4 lever and won't be able to get over into reverse position (of 4 speed pattern) when going into 1st. position of four speed but is now what would become your reverse as a normal 3 speed floor shifter.
I did it on mine with my Competition Plus that was on the M21 Muncie I had in the truck for years. At least until I came up with a Hurst 3 speed shifter that I traded a buddy out of. I left the reverse rod off and think I wired the reverse lever up so the shifter wouldn't pop over there making it work like a regular 3 speed floor shift.
I found and bought a Mr. Gasket V gate inline shifter. I suppose I can use it on 3 speed just like other ones. If not maybe I could trade it for something else.