You should be able to use some truck size jack stands and a hydraulic jack. Jack up each corner from the tranny crossmember (thats what I do) and place a jack on the front frame rails and crossmember. Do that on all corners till its high enough. If the jack doesnt go as high as you need use some 2x4's or something to help place between the jack and body.
Since your replacing the floors you probably want to support the mid and each ends to keep the unit-body from sagging in the middle. You need to keep as much support on it as possible when removing the pans. Make sure its level, use a level or count the notches on the stands if they are the same kind stand.
I saw a video once..cant what it was called or where it was on...on replacing floor pans in a mustang. You dont want to just go chopping out the floor and welding in the new. The factory spot welded the pans to the support beams, I think you can drill out the spot welds from the support beams then cut the perimiter edge and mig weld them in.
Here is a web site of what im talkin about.
Boyd's Mustang: Floorpan replacement
Good luck!! make sure you secure the car up in the air safely...you can get hurt or killed if you makeshift something to go under it to hold it up.
