I just did my '98 379 Unibilt the same way. Removed the sleeper, moved the rear air bag mounts from the sleeper to the cab.  Just made a cardboard pattern, took it to the local sheet metal shop along with the window I was putting in.  Box of 100 rivets later it was in.  Don't want to ruin the sleeper back wall as I'm going to try and sell the sleeper so I need to glue on some interior material. I've got about $500 into the panel, rivets, window and window gasket.