Honestly, you are best tracing the existing floor pan before removing it...it sits on the lower firewall at the front(floor-firewall-extensions they are sometimes called...but they are part of the firewall) and on the shock mount panel at the back where it drops in a radius below the rear seat(you can see some of the details in my daily driver build thread...replaced the vast majority of my floors in various patch panels). Assuming that the replacement full floor pan is the same(and most of them probably are) you should be able to realign it off your tracing marks. The only real concern you should have is keeping the car braced while you do the surgery...pan alignment itself will be easy.
P.S. Just for reference...the back lip of the pan sitting on the shock panel ledge(minus where I cut it away to repair some rot...also a useful picture because it shows the frame rail location at the back.
What you will find is that if you verify measurements on the new vs. old pan from rocker to rocker, there is only one way it fits in width-wise...and front to back if you trace your old pan before removal and verify those measurements too, you wont have any issue, always assuming the car is well braced before removal(probably the biggest reason I chose the piecemeal approach...I was able to cut out one section at a time so I didnt have to do that...good enough for a daily driver...not ideal fort a show car unless you spend a lot of time on grinding butt welds perfectly)
Last edited by wicked93gs; 07-01-2019 at 10:54 AM.