start looking around page 448 of 736. it shows an L bracket bolted to the firewall with (2) studs on the firewall and then a through bolt with rubber grommet at the connection from inner fender to that bracket. I could easily see those elbow smasher studs being ground off during a rebuild or customization. anyway, here is a link to the assembly manual. if you don't have the studs you could easily get some dimensions from somebody on here with a stock firewall or simply look on the inside floor area for a reinforced spot where the studs went through to a plate or something. i can't exactly remember how that part looked but know you should be able to find the spot from the inside if "repaired" on the engine side of the firewall.
https://www.trifive.com/55-59Assy.pdf