So all your going to do is swap the cam? No head swap or intake swap or MAF or TB swap? IF nothing but the cam is going to be swapped then there will be minimal gains. It is hard to get air into the 302's. The cam won't help with that since the TB and intake are bottle necks. The heads without work don't flow all that well either.
Like blue is saying if you don't have the heads and intake then you will see minimal gains with the cam, RRs and TB. The lower intake 5.0 HO intake is the first bottleneck on the 5.0s. It doesn't matter how much everything under it flows if the intake won't support it.
Now if you already have the intake and heads then I would imagine you would see gains. If you don't have the heads and intake I would wait until you have them to install the cam. That way you can do it all in one project. Keep in mind that the stock 5.0 cam is a pretty decent stock cam and there are lots of guys going plenty fast NA with the stock cam. How much you will gain in hp, mph and lower ETs I couldn't make an educated guess at though.