It really depends on the condition of your car as well as the condition of the fastback as I wouldn't want to trade a coupe that's worth potentially double the price of the fastback. Especially when the fastback needs paint, a drivetrain and so forth as that will only add considerable amounts of money to the equation. I think that fastbacks are always going to bring considerably more money than coupes will, when they're in similar condition to each other. I think some pictures, better descriptions and so forth would help determine potential dollar figures as to what each car might be worth. But if your car is worth say $15k-$20k and you trade straight across for the fastback then you've already got $15k-$20k into the car, then figure at least that for paint, and finally it depends on what you want to do for a drivetrain. You could still do it and be money ahead, but I think that it will be very close, and that the better idea might be to do a trade as well as have him give you a few bucks. Ultimately it depends on what your goals are as well as what your budget is, or if you don't care about being money ahead or if you honestly just don't care what it costs to do so. It seems like if you're posing the question that you're likely leaning in the direction of doing the trade, but I'd personally need to see the car in person and need a few more details about it. If it could be advantageous for you in any way then I'd absolutely do it.