I have a 79 Bu with a factory bench. I'm running a ratchet-shifter behind a stock Chevy roller motor.
I ride with the seat back about three inches from full front. I was able to mount the shifter to the floor with no issues.
I had some latitude in where to cut the tunnel because the shifter works off a cable.
It's not exactly where I want it but it is close enough not to be a bother.
Besides, I only ratchet when my foot suddenly gets heavy.
However, the shifter of a stick has to mount to be mounted based on how it connects to the trans.
I would measure the distance from the back of the bell-housing to the spot on the trans where the stick will need to poke through the floor.
That will give you an approximate location.
Then you need to estimate how high off the top side of the tunnel your shifter will be and allow some backward tilt of the stick in 2nd and 4th gear.
Remember, you can play games with the height, bends (contours) and mounting angle (forward or back) of the shifter stick, so you can make it fit but at the end of the day, you have to be able to shift every time you drive the car without it making you crazy.
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