Danny,
Pull the stems out before you solder so you don't melt the rubber 'o' rings and seats. When you get in from the back wall, you'll probably find a 2x4 backer the old stems are mounted on nailed between the studs. You can easily remove this by cutting the nails with a sawzall or just a hack saw blade. Mount your new stem assembly to a new 2x4, then solder in (use wet rags on anything you don't want to burn), then just screw the 2x4 backer to studs when you're done.
Piece of cake [img]/forums/images/icons/wink.gif[/img]
Hope this helps.