302 or 351C?
 

I have a 67 with a 200 in it now, so obviously my first step is to get a better engine. I guess my first question is how much am I going to have to change over with the 351C, that I can leave the same with the 302? The other question is will the extra work be worth it in the end? I don't expect a list of what I'll need to change, just an idea of what I'm really biting off here. Thanks guys.

Either way, you will need a new transmission, driveshaft, rear axle, and you will want to switch to 5-lug hubs from the 4-lug that comes on 6cyl cars. I would recommend the 302 over the 351C just because performance parts are much easier to get for the 302 due to the 5.0's popularity. You can get aluminum heads for a 302 very cheap now.

You may want to think about a 351W, It uses the same heads as a 302, but with some extra cubes.

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67 Fastback - Arctic White Pearl paint
351W ,Trick Flow Aluminum Heads, Edelbrock TorkerII, Comp. Cam, Performance Automatic C-4 Trans, 3.55 gears, Front Disc Brakes, 1-1/8" Fr. 3/4" rear sway bars.

My 351W Fastback
http://www.mustangworks.com/mustangs...1245-slot2.jpg

The common wisdom on the I6 to V8 swap is that if you want a V8 car, then start with a V8 car. There are considerable differences including radiator, motor mounts, fuel lines, wiring, transmission, drive shaft, rearend, brakes, front suspension and steering components, etc. Either sell the I6 and buy a V8 car or buy a V8 parts car and change components to the I6 car.
If I were going to the trouble to put in a V8, I would go for a 351W.
Is it worth it? Probably not if you're planning to sell the car, it would be worth less than a stock V8 car in the same condition. But if you're looking for the fun factor and DIY pride, it can be worth it.