Road race and significant other bench seat
If the upholstery support comes from the metal springs, then to get anything laterally supportive by dropping the middle will require cutting and welding.
If the support is just done by building up, your position in the car be altered. The seat center can not be built up. Same with the back middle. Otherwise your helmet will be in the headliner, and the seating adjustment further back so your feet will fit in. The bolsters may hit the armrests when the seat is folded forward.
One solution will be to lower the whole seat, but this will require notching and boxing the center to clear the transmission tunnel.
How easily that can be done will depend on the seat's construction.
If the upholstery support comes from the metal springs, then to get anything laterally supportive by dropping the middle will require cutting and welding.
If the support is just done by building up, your position in the car be altered. The seat center can not be built up. Same with the back middle. Otherwise your helmet will be in the headliner, and the seating adjustment further back so your feet will fit in. The bolsters may hit the armrests when the seat is folded forward.
One solution will be to lower the whole seat, but this will require notching and boxing the center to clear the transmission tunnel.