Hi Bruce
Had to go out to the shop to get the name of the best weld primer I've found seems to work best for both mig and pinch/spot weld.
SEM 40783 Copperweld
As to sliding lower panel under upper, I use several ratchet straps to pull it tight to the body, and several more to pull it up under the upper lip.
Lot of photos of restoring my HUP on my site, quite a few large files at higher resolution at
http://www.canadianmilitarypattern.c...Y%20PHOTOS.htm only problem the page takes for ever to load its one of those start it down loading then go get a cup of coffee.
Cheers Phil