The mounts that bolt to the engine are from Chassis engineering. Unfortunately they have gone out of business. You may be able to get them from Tom Lowe at 12bolt.com. The mounts that are welded to the frame to hold the cushions are just some 2 1/2" square stock I had laying around. Not sure what piece you are talking about on the drivers side????????? What #post were you looking at?
Heres a set on Ebay that are similiar to mine....
https://www.ebay.com/itm/CHEVY-GMC-2...-/371532040945