Same as you know what, just a different pile.
Pg 2 will interest you but pg 1 is the nuts and bolts and will warn you about SEEMS OKAY. The difference being I replaced the front in four pieces with a 1/2" lap joint. Take the total width of the coach, add the 1 3/4" insert into the C channel, divide by two and add 1/4" and do a 1/2" lap joint.
As for finding where floor grid is, you can always drill up through floor along side a piece of framework and then add half of framework width to get centre or even easier....the existing nails will tell you where the framework is.