A full floor is just about the easiest panel to replace.
A few tips.
Make sure the edges and surface are clean and straight.
Use finesse. You don't want to go nuts with an air chisel or an electric grinder with a stone.
Use drill screws to hold it all in place. Not many places to clamp with vice grips other than the firewall pinchweld.
Make sure you get good weld penetration into the frame rails and torsion bar crossmember. I do this by taking a screw out, drilling the 3/8 hole thru the new pan and partially into the lower rail. That way the weld will heat the lower enough while also filling in the drill screw hole which ensures a strong weld. Practice and you will get good at it.
Weld thru primer is a good idea in between but clean away at the welds first or it affects the weld quality.
Scuff the whole bottom of the new pan with a red Scotchbrite pad first or you will be doing it from under the car later!
Try to use as many plugwelds as the car originally had in the same locations.
Check if the new pan has the wire straps for the speedo cable and trans wiring and transfer them over first too.