another rear window question

[64_twins]

Hey guys, I've read most all the info on replacing a rear window but I'm thinking I may not have an original in mine. If you place the gasket in first and cut it to fit and then place it on the window there is at least a 2-3 inch overlap. This makes it virtually impossible to put the glass in the gasket first because the window won't stay in the gasket if I don't fit it flush. I noticed when I took the window out there was quite a space between the edge of the glass-gasket-to cab frame. Can someone give me dimensions of a small rear back glass? Any suggestions would be helpful. Thanks.