Thanks Bomp!
More details on the seat risers, here the 5/8-11 hex nut is welded inside. The tacks at the top are merely to hold it centered within the hole to maintain our correct C-C hole spacing. It gets welded inside as well to keep things anchored in place.
The 5/8 hardware was chosen as the factory seat bracket on these 2002 Monte Carlo seats had a hole that large existing already. The front of the seat bracket actually had another bracket underneath that was held in place with a solid rivet. We drilled out the rivet to remove the obstacle (so the rail was flat across the bottom), and opened the hole up for 3/8 hardware.
Test fit....
Still need to fill in the flange corners on the front and this one will be ready for SPI epoxy primer to coat all the surfaces before it gets welded in place..