I just pulled my 39 PU frame apart about 2 months ago. First remove the frame from the cab. It is VERY tight as the square plug is rusty and the 10-32 machine screws are rusted into the square plugs. I used a 50 - 50 mixture of ATF and acetone and soaked the joint and screws liberally for about a week before I saw any movement. First get the screws out in one piece if possible to avoid more work down the road. Since one of my glasses was broken, I was not afraid to put some heat on the frame. I used a block of wood and a hammer to persuade the upper and lower sectons to part. I ended up breaking the good glass by applying heat to that side. I also quenched the frame after the heat to break the rust bond. It took several days of heat, quench and soak to get the upper and lower frames to part, but they will part. As an aside, the plugs are 5/16 square and I had to have a 3/8 bar milled down to 5/16 and drilled and tapped to 10/32. There is a slight bend in the middle of the square plug, put the bend in before you drill and tap the threads. Nobody ever said that working on these old cars was going to be a piece of cake.