I don't agree that a buyer would kill you on the price for a converted car that had an I6 VIN. The I6 cars are not really worth all that much to begin with unless they are convertible or maybe fastback.
What I mean is, a guy buying a car will balk at paying V8 prices for a converted car with 6 cylinder VIN.
As far as mileage goes. My car was a daily driver for a few years and got about 16-18 mpg. Now it gets about 16-17. So, the mileage really isn't any different. Of course I have a T5 now.
Strange, I had a couple of 66 Sprints, and the auto convertible got 20-25, and the manual coupe got a few better than that.
Also, It doesn't matter what exhaust you put on a 200 it will sound like a 200 not a V8.
Wanna bet? Dual headers and GT dual exhaust sounds like a stock 289 GT, maybe a little better.
If you want to tackle the project then I say do it. If you are happy with your car and really just want a bit more pep, the you should probably stay with the I6. You could always swap out the 200 for a 250.
The 250 uses the same bellhousing as the 302 V8, so you'll still have to change the trans, minimum.