How much can come out on a wrap around rear window chop????

I have a 1951 Buick that I would like to chop, but still use the stock rear window. What is the most that can come out and still look good? I don't want the rear sail pan rising up, I would like to keep the flow of the stock belt line.

I have read http://www.jalopyjournal.com/forum/showthread.php?t=381801 which Yaril chopped 2" out of (I think). Has anyone gone with more. I know that there is a limit with still using the stock rear glass, and just hoping that someone can help me out.
Thanks

 

I am not too sure you can do much more than that without getting into some serious headaches. You are essentially tipping the rear window forward, which raises its tail. The rear window frame is right on top of the belt-line trim. You pretty much have to move it with the window, and re-work the intersection where it meets the body. The more you tip it, the more abrupt the transition between the deck and the glass will be.