Just my opinion man, but I say go for the 351. I mean you definitely don't intend to get rid of the Mustang do you?! If you were planning on parting with it at some point you should leave the stock engine in but if you are like me then you want all of the power you can get. And although it will caust you a heck of a lot more to put the 351 in, just keep reminding yourself of how great it will be with this easy to remember magic word:

"...power"

or even,

" POWER! MWHAHAHAHAH!!!!"

Though you will wanna make sure no one is watching or listening to you if you do the maleviolent laugh....