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
