Done right, you can get a 351w under the stock hood. Then you wouldn't have to change the body at all.
A 302 and a 5.0 are the same motor. Most people use 5.0 to refer to the fox body cars, even though it is a 302w, just like the motor in a 68.
A stroked 302 is good for 347, but you can stroke a 352 to 408. Not wouldn't that be fun in your 66? The 351 is a stronger block, and if you are really concerned with racing, stroke it and build from there. Thats where the most power will be from your options.
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1997 Mustang GT "The Freak" - 13.80 @ 101.70, 2.07 60'
1995 Honda VFR750 - not much @ really fast (actual data pending.)
1964.5 Mustang 289
Rice Haters Club Member #13