Well lexan is no way to do windshield in a car you intend to drive frequently other than a race car.

Just lengthening the roof will look like crap. The pillars usually need to be tilted back for the right look. As well, they need to be tilted in OR the roof needs to be widened.

And then its not just a matter of cutting the existing glass (if it can be cut) because the curvatures may no longer be correct.

I think this is something you want to do a LOT of research on and have the answers before you make your first cut.

FWIW, I was at Carlisle a couple years back and there was an old Caddy there with a drop dead chop job. The car was for sale for $2500. and was easily worth that. The problem ... no glass !!

Keep in mind that the side glass will also be an issue.

One thing you notice reading about any chop job is they generally mention the windsheild being out of a something or other. In other words, they found a windshield that looked right for the job and they proceeded to build around it.