The tubes on both ends bolt up to existing factory holes in the frame. Note that the 81-up frame is fairly different than the 73-80, so if you're trying to use the tubes on an older truck you'll need to do your own drilling.
The rifle holders, such as they were, were two rectangles of sheet metal formed and spot welded to the floor, and then the two twist-holders up top. These were just behind the driver's seat and shoulder belt. The photos I took when I bought my M1009 aren't very good, but they're a start. Looking towards the rear, driver's seat on the left, shoulder belt on the right:
And from the rear we can see the twist holders a bit better:
The only pic I have of the front tubes is from straight on, so it's not real helpful, but at least gives you an idea (and it's with the bumper off):
I was installing a Warn receiver hitch on the front at the time.
I'd offer to take more pix, but I no longer have the truck. Sorry
-- A