To get a 302 to make 500 HP is going to cost you more than to make a 351 make 500HP.
302 N/A 500 hp is out of the question unless you put a lot of money into the block and engine to rev it real high.
A rebuilt 302 with all the goodies, heads, intake and cam, fuel system etc... N/A should make 365HP at the crank. Maybe a little more if you match parts real well. If you boost that then you can definately hit 500 at the crank. But again, a 302 block is weak and is only a matter of time before the block splits at that power level.
A 351 is a stronger block and a better foundation for your goals. A good 351W buildup in my eyes is a bored out 351 (357) and possibly stroked and a good set of heads (AFR205), really good balanced forged rotating assembly and a good matched cam.
In the end youll be spending less money to make a 500 hp engine with the 351 as opposed to the 302. Why do more people like building up 302s? bigger market for the 302 and more people know about the 5.0 as opposed to the 351w. But for anyone that wants to make serious power, 500+, the 351w is the best option.