do i need 2 know some important facts before i replace my floor pans on my 65 Impala SS?

Nothing major. Maybe get a repair manual...chilton or haynes if they make them for that year. I think they do.

I found it best to replace one section at a time. If you can leave the factory bracing in place drill out the factory spot welds and plug weld the new panels to the old braces. The trunk floor is harder to do.

you dont want to cut the braces out unless your changing them too which requires the body to come off the chasis, us a spot welder bit to cut spot welds