The hard part is not going to be what you can get the new vert for...the hard part is getting a decent price for your coupe. Remember, the profit marigin on a Mustang is rather minmal compared to other cars. I dealer willing to go a couple hundred ove rinvoice is motivated....if that same dealer is willing to give you several thousand more for your trade-in than Carmax...you have a highly motivated seller.
You are not likely to get much better terms anywhere else. You may get another dealer to give you either a better price on the new vert, but less on the trade-in or give you the vert for a higher price but give you more for your coupe.
Obvioulsy, you best bet would be to sell you coupe private party, then hammer the dealer for a vert on the lot at the end of the month. However, that is a lot of timing issues. Sounds like you have decent dealer that you are working with, I would run with that.
The only question I have is $500ish for DOC fees? Maybe for delivery charges (to deliver from factory to dealer) but not document fees (should be like $35) | 
