This document doesn’t say much about mounting shoulder harnesses. I think your idea of adding a sturdy tube behind the seats and below the rear window is a good idea. Judging from what I see on new cars, the ideal position for the top mount of the belt should be just slightly higher than the occupant’s shoulder. You might need to add a riser to the cross tube to get the shoulder belt attachment to this height, and keep the cross brace below the bottom edge of the rear window.
For the cross brace, I would use good-sized round tube (perhaps 1 5/8-inch OD, with a 0.090-inch or .125-inch wall thickness). There are many ways to mount this tube, but welding it to the body sheet metal is not a good idea. I’d probably put bends in each end of the tube, so the ends come up to meet the rear door jambs. I might even weld a piece of 1/8-inch steel plate to the door jamb to reinforce the attachment of the tube. With careful design, some or all of this could be concealed with the upholstered panels to the sides and the rear of the seat.CT