Rear panel / crossmember

The aluminium / burmbrite panel that is under the rear door on my defender td5 2002 is rotten where the 5 pairs of bolts go through it and hold the rear body down to the rail attached to the steel rail attached to the top of the crossmember All but two ( the outer ones ) of the bolts holding the steel rail to the top of the crossmember have pulled through the top of the crossmember as well I can get the alu panel 330720 I was going to stitch weld the steel rail to the crossmember and then cut out the alu panel and try and fit the new one with pop rivets. Anybody tackled this before getting at the outer edges might be a problem with the quarter panels in situ . At the minute my rear tub jumps about an inch in the air over bumps