Without changing internals, it will be a bit harder to get to 300 than if you started with a '67-70 engine, but, it's still doable as the other responses laid out. I'd really be more worried about you using that 7 1/4.....even without the HP boost! Blowing a rear end can have all sorts of bad results, from over-revving the engine, to transmission problems, to out of control issues, depending on what breaks.