Im gonna have to go pick the 71 charger from the body shop cause im just not happy with the performance (or lack of)
I need to put in the floor pan (most of it is cut out now) with a new 1 peicse pan
after all edges are dressed up how hard is it to get to fit and weld in?
I have a 130 wire welder but havent used it a lot
any one fit a pan on a 3rd gen charger? what problems should i expect/look for?

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.

Will i have to force the trannyccrossmember up into the floor?
Will there be any air gaps where the floor just not want to lay flat?

No to both. As stated above, use self tapping sheetmetal screws to hold the pan down tight everywhere

I just got done recently replacing the floor pan in my Duster.TAKE your TIME and think things through for each step.Use weld thru primer between the floorpan and the frame rails,etc.Plug weld where the original spot welds were,approxiamatly the same distance apart. Use plenty of vise-grip type clamps where possible.Support the car under the rockers near the front to take the weight off the frame/torsion bar area......P.M me for my phone number,I can walk you through it if you have questions........YOU CAN DO IT!!!!!!!!

You can manage by doing lap welds and have it turn out so you have a floor again, but to do a good job to where it's not noticeable requires butt welds.

Just saw that you have a one piece pan... no butt welding needed. You're good to go.