What I am going to do on my 48 Chevy is lengthen the roof AND lean the posts back. In other words lengthen the roof about half as much as needed, then lean the posts back the remainder needed. On most chops such as the "AD" Chevy if you lengthen the roof the windshield looks like you leaned if FORWARD. The reason for this is the distance from the top of the glass forward meeting an imaginary vertical line from the bottom of the glass becomes shorter than when the top was stock! Soooooo, it effectively makes the windshield look more vertical than when stock!
If all you do is slant the window back then the windshield looks just that, like it is slanted back and doesn't match the factory lines.
Lengthening the top half way and racking the posts back the other half gives it lines more like the factory would have if they were to have put the top lower to begin with.