I would recomend the 351w if you are going for power. One of these will provide about any power that you could handle. Best of all, it'll fit in the tight confines of your mustang. 351 blocks are getting a little harder to find now because they werent really used as much as the 302s. I found a block on ebay for $50. You just have to look is all. THe aftermarket can provide just about anything else. Before you know it, youll have a killer fastback. If you are going for practicality, maybe the 302. It'll provide better gas mileage. Both have so much aftermarket parts for them.
If i were uou i would contact PAW, thet make a 420/427 small block winsor. with cheap components, 400M crank(4-1/8 stroke), mopar 360 rods (6.123'), Keith black hypereutectic pistons, all lodified slightly to fit. and the price for a shortblock, with cam and balancing is 2495. and a complete long block is only; 2995,- with 351 w heads. just add a proper intake and all your stock hardware. and your the proud owner of a 500 hp mustang.. that idles and runs on pump gas
luv 2 run has a good point. a 347 is just 4 cubic inches smaller than a 351 and about just as expensive. And you can get just as much horsepower from it too. An added benefit would be that you dont have the extra wieght penalty of the bigger 351 block, so you could preserve handling.
iluv2run is right if you want an engine that is a fair price and has a good amount of power go with the 347 stroker. there are many modifications that can be made for show or for race to this engine. and as far as cubic inches goes 347 of em is pretty good since stock was only 289