Roughly your gain is somewhere around 10 HP per pound of boost. Roughly.
I would bet that with the decent combination of top end parts, the added displacement and his higher compression ratio, gains would be closer to 15hp per.
I wouldn't be too concerned with blowing anything up with a little boost either. I'm assuming that since it’s got a bore over and the pistons were replace, you replaced them with forged, or at the very least decent hypereutectic slugs, did you not?
In which case, the pistons should hold up. What usually pukes first when boosting a 302 with compression is the head gaskets because of the inferior and inconsistent clamping force of its 7/16" head bolts that allow the head to distort under pressure.
...but since you're working with a 351W that has stronger 1/2" head bolts, you're not likely to experience that issue.
As I stated....5-8psi (which is really going to depend on pulley size and flow capabilities of your top end and exhaust....so don't just use boost figures as your guide) with conservative timing and generous fuel curves your engine could easily handle a little forced induction and would reward you with some decent gains.
I recommended the nitrous because quite frankly nitrous will be the cheaper set up (provided you're not going through a bottle a day) and compression is actually a welcomed advantage when spraying.
In any case, if you cheap’d out on build components like pistons, bolts/stude, or head gaskets, etc and aren't sure about fuel system suporting components (injectors, pump, fuel lines, etc)....then you limit your engines capability to handle power. It doesn't matter which power adder you're using. In any case a compitent dyno tune is going to be a must for either set up. No half assing it with FMU's.