rear frame replacement

it looks like im picking up my second duster this week. this one is a 73 with the leaning tower of power, and an auto. body looks pretty good other than the quarters being solid bondo. the left spring is pushing through the floor, and the right is weak and starting to bend. frame rails on both sides are junk, but only like the back foot. the trunk pan looks good except the spot thats pushing up from the spring. my 74 has had an area about a foot long replaced on one side with a piece of 2x3 tube. Im wondering if anyone here has done something like this? Ive done aot of chassis welding, was a cert welder for years, so im capable of doing the job just wonder if anyone has any tips to save hassles.