That is what I did, got a random drop from a friends metal shop of 12ft, just cut it into 6 ft pieces. I only have gotten one side down, but cutting the front and rear pieces about a 45 degree angle leaves me about 8" or so of a scrap piece. I did however trim both the fenders down some. The front sticks a couple inches into where the fender mount goes, but I did it so it is roughly flush with the fender opening. Then the rear stuck out 1/4" or so from the end of the old rocker panel.
In my case also I am doing some heavy gauge steel as floor replacement panels that goes right to the new rocker panel. Still will probably put a couple of "cross supports" in there to the main torque rails, but will decide once I get the floor pieces welded in lol. Project has taken WAY too long at this point so I want to try making as much progress as possible in the next month or two, hopefully to have it done by August.
Not sure how far up you are planning to take the bottom edge of the sill to mate up with the rocker tube, I unfolded the recess in my old rocker panels after where the door edges are, and bent it down up to where the inside lip is. Welded the whole length to the rocker tube with a slight angle further more and this way I can run unmodified stock doors on it and still have a tiny bit of room (for a liner coating I am still doing) and it will drip away any water that gets near it. I thinkI still have some pics somewhere of what I did, can upload them if you are interested in checking them out...
Paint