Yes, you need to measure from a straightedge clamped to the front of the firewall (front of dash.) Every front-to back cab dimension is referenced from there. The FAM Section 1 Page 1.07 has the cab dimensions, including the body mount hole positions. The body bolt is 13.19" behind the firewall. And 23.5" from the cab centerline. For the side to side positioning of the cab, I would measure between the lower hinge bolts and weld in a temporary brace to maintain that at the correct width. The floor panels I put in matched fairly well as far as the body bolt depression. As with any replacement body panel, some adjustment will likely be needed. Also the joint between the floor and the toeboard panel is 8.47" behind the firewall.
