My part #20-0735-81 from Steele Rubber arrived. It's a nice piece, USA-made vulcanized rubber, heavy and soft. It is a loop (as you would expect) with no corners and not a straight piece of gasket. I got it for $75 from Steele's ebay store.
I measured 20-0735-81 using a flexible tape measure and it is right around 118" around the perimeter.
I measured around the rear glass of my L320 and that perimeter distance was between 96" and 98". So the Steele gasket should definitely be long enough (that was actually obvious before measuring) and will need to be cut down. I see no reason why the profile of the Steele gasket wouldn't work.
I also measured around my truck's front windshield. I got roughly 117". The front windshield gasket looks similar in profile to the rear one on my truck, so 20-0735-81 may work on the front as well. In fact, the measurements are close enough that it may be a fit without cutting down the Steele Rubber gasket. That's probably too good to be true, but I will try it.
I will try to remember to post back here after I've actually done the install of this gasket.