I'm reading several misconceptions here (which I think are just lost-in-translation incidents). Your interest rate and length of loan on your current Mustang fail to matter in trading in the car. What matters is the book value of the car on trade in versus the principal balance of the loan. What do you owe, and what is it worth? If you had 25% interest, it wouldn't matter on trading in the car because there are no penalties on simply interest loans (which is what you have).
The confusing part is what you wrote: the 19,000 and 26,000 thing. That doesn't compute on a 6% loan for 60 months and a 350 payment. Regardless, it isn't my business. To know what your car is worth, take a look at NADA.com to book it. Is trade in value higher than what you owe? Start there on deciding on a GT.
Just so you know, 6% is not a great interest rate. Rates vary based on credit, but dealers are allowed to sell you any loan you accept. For instance, you might qualify for a 3% loan yet agreed to a 6% loan. Guess who pockets income off that? The dealer. That'll really cost you. Call a credit union or bank and investigate what sort of rate you qualify for (based on several factors like income, debt, length of time on job, credit history, etc).
If you have any questions OP, put them out there.
2014 Pony Package convertible l Deep Impact Blue on Black l auto (current)
2014 Ram Big Horn Crew Cab 4x4 l White on Grey l Chrome 20"s l Navigation l Hemi (current daily driver)