If you can get out of the late model without being upside down, and plan to pay off the 1968 quickly (so you can keep it and get another new GT as your DD), I'd say do it.
If the 1968 would have to be your long term DD for financial situations, I'd stay with the modern ride.
2011 GT Prem Kna Blu
"Quick like a Snake, light like a Fox. This remains the goal."