what is the easiest way to replace the floor pan

With a lot of tough work. There is no easy way. Get ready for weld fumes!

i have the body off the frame but its not light

Well, depends on how much of the original floor is bad. If you have the typical rust in the floor wells, an appropriately trimmed repair panel butt welded in place of the original one is the slick trick. Basically cut a repair panel just larger than the damage, lay it in floor pan and trim the damage away to fit the repair piece nd butt weld. If the damage is extensive, includes factory seams or pillars then the job gets tougher. Do a search here of search the web. There is a lot of information on it. Hope this helps...

Cam