Seat mounts and harness

I have a question on my drag race build. It's not an a body but it's a 78 dodge d100 step side. I have mounted my plastic racing seat where I want it but the problem is the mounts don't line up with the crossmember on the bottom side of the cab. Being it is a pick up I cannot go to the frame and there is only one framework piece on each side where the original bench seat mounted. How can I mount the seat and be legal with the nhra rule of the seat mounting?

Another question is how to install the harness with the crotch strap and shoulder belts. Right now I have the lap belts mounted to the factory bolt holes for the original seat belts. With no roll bar what is legal to mount the shoulder belts to? I have the doubled up with the lap belts on the floor behind the seat. The crotch strap I have bolted to the floor sheet metal with big fender washers. How can o do this legally and safely? Will I plate under the floor with the crotch strap and seat bolted to it pass tech?