This is an old thread but I am currently wresting with the same situation on my 49 3/4 ton. The driver side window popped out of the track. When I removed the window the lower half of the front channel guide was laying in the bottom of the door. The whole thing was originally spot welded together and to the door frame. The U-shaped bracket at the bottom is still spot welded to the frame but the spot weld attaching the channel to the bracket is broken.

I have ordered a replacement assembly from Chevs of the 40s (item# 3685369). It is a bolt together/bolt on kit that includes the channel, the U-shaped bottom bracket and mounting screws. Mounting the top of the channel will be a snap because that area is accessible from the window opening. The bottom part is the problem. Since the bottom bracket is still securely spot welded to the doorframe it would be nice to just bolt the bottom of the channel to it instead of using the replacement bracket from the kit. That bracket is in pretty tight quarters with limited visibility. Has anyone else figured out how to drill holes in that bracket for the channel mounting holes?

1949 3600 w/ 57 235