Seat Height: Replacing the power units with manual seat tracks from a '75-'77 Corvette will lower the seats enough to allow sufficient head room. These are usually readily available, and we found them on eBay. Other years may also work.
Seat Rake: we found that making four "risers" gives the right angle to make the seat bottom comfortable and hold you in place. These are positioned at the four C3 seat track mounting points for each seat. The front risers we used are 1 & 1/4-inches tall and the rears are only 1/4-inch tall. Depending on your height and personal preference, you may want to experiment to achieve the rake that feels best to you. All four are angled at 4 degrees to allow for the angle of the seat tracks. We used 1-inch-diameter solid aluminum for the risers. Each riser has a 3/8-inch hole in the center for the mounting bolts, which are 3/8-inch x 6 Allen (socket) head bolts. The front bolts are 2-inches long, and the rears are 1 inch in length.
The seat risers also provide another function by allowing the clearance needed for the mechanical adjuster mechanism arm on the seat track, as well as clearance for the cables used for the seatback adjustment.
Mounting: There are two areas requiring mounting: the manual seat tracks to the seat frames and the seat tracks to the car floor. The former is accomplished using the risers and bolts mentioned previously. The locations of the new holes you need to drill and tap are shown in the accompanying diagram. Basically, you need to drill and tap two new holes at the front underside of the seat frames, which are 1 & 9/16-inch in from the outer edge of the seat frame and spaced 14 & 3/4 inches from the rear mounting holes. The rear mounting holes use two of the holes now occupied by the innermost rivets that are at the rear of the seat frame. The easiest way to remove the rivets is to use the appropriate size punch and drive the inner portion of the rivet inward. Then, drill the rivet center with a 1/4-inch drill bit to remove some of the rivet head and cut the head of the rivet off using a sharp chisel. The inner portion of the rivet will go inside the seat framerail, but those pieces can be extracted through the center hole in the crossbrace. We used a vacuum to suction them out. Then we drilled the hole with a 5/16-inch drill bit and tapped the holes for the 3/8-inch x 16 socket bolts.
Once we had the seat tracks mounted, we installed the seats in the car to get the position we wanted, making sure we had the seat tracks fully to the rearward position and marked the holes we will need to drill through the floor. Midyears have a steel plate riveted underneath to accept the stock seat mounting bolts, but we found the holes didn't line up, and we preferred to increase the thickness of the mounts. We made three new plates for each seat from quarter-inch-thick steel, which were bonded to the underside of the floor. A single front plate is used for both front seat track bolts and measures 17-inches long by 1 & 1/4-inches wide. Two smaller plates were used for the rear seat track bolts to better fit the contour of the floor which measures 3 & 1/2-inches square.