I found the thread on installing the side windows (not the little triangular ones, but the big ones). They state to use urethane caulk (3M Windo-Weld # 08609) to attach them to the rail on the window mechanism. However, the factory install uses a rubber u-channel.

I'd rather use the u-channel, since it doesn't appear that using the caulk allows for any adjustment fore and aft after it cures.

My windows were taken out to paint the car so I have no reference for putting them back. The window install post stated to measure about 4-3/16" (106mm) from the front, but what is the tolerance? I'm thinking there could be some variance in location plus or minus 1/4" (6mm). I thought of setting the window in place, but I won't have the triangle window in, so there will be nothing to reference the location off of. Any ideas on solutions?

Where can I get the u-channel?

Also, is the u-channel glued in or just friction-fit?

Do you have the rubber U ?
It's friction fit if the metal part is intact....they do rust and loose grip.
without the rubber you will have rig something to make the
rail stand straight on a mechanism that is already marginal.
early window parts can be found

The metal parts I have are in great shape. I only have one rubber channel. I thought I might try and find some rubber sheet and make another channel.
The 3M caulk is $31a tube, so I thought I'd try an alternative first. I want adjustability as well; the caulk doesn't give me that.

I used urethane when I did my drivers window. I got the angle wrong and the window sits high in the front so when rolled up it doesn’t seal correctly. I didn’t realize there was a rubber u channel used at the factory. Make sure you get it lined up and sealing good if you glue it.

$99 a set got the new tops too
https://900designs-container.zoeysite.com/t...window-seal-set

Can you post a picture of the rubber u channel?

Here's the rubber U-channel:
Click to view attachment

It is 2.5mm thick.

I found another one! It was stuck in the metal channel. The one I had was stuck on the window. I was able to carefully pry it out. I'm going to reuse both of them. They appear to have been secured with some caulk of some sort as there is still residue. I could rub most of it off with my fingers. I will obtain some butyl caulk and secure them with that.