Mine is apparently very different. It's all one piece going as you originally said from the front up across the top and down the back.

To start I tried to bend it by hand and then I fed it into the channel and got it close. I then raised the window/glass frame into it as I tightened the screws on the trim pieces on the inside of the door. This kept the channel from buckling since it was trapped between the door frame and the window frame.

I haven't had a chance to drive it and hear how air tight it is but it certainly looks like it went in as good as it could.

'69 1/2T stepside, Sea Foam Green, 250 L6 w/ 3 on the tree, not an option on it, in the family since new.