side passenger window install question, rubber piece or urethane caulk? Options

[914 RZ-1]

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?