Hypothetically speaking ,.. What mods would be necessary to make a stock 350 reach 500+ hp/TQ ? I'm tired of looking for the 455 and am considering just keeping what I've got. Rear end & suspension are stock btw.

 

Can you not find a 455?
In order to get 500 streetable go from a 350 you'll need to either go blower or spend a lot of money, most likely 10k+.

 

that kind of horsepower from a small block requires 6800+ rpm or forced induction, or funny gas. So the best (read most expensive) of everything is required. FWIW, I have almost 5K in my 455 and I'm only making about 480hp.
One key to getting satisfying performance out of a small block is to get it into it's powerband quickly and keep it there. That means transmission and gearing must be selected with this in mind. It might be more realistic to shoot for 425 hp and just make sure all other components are optimized.

 

I know the 455 is ideal but have not found any here in HI. I won't go back to FL for some time , so I just wanted to know my options.

 

What year vehicle? Stock could mean 2.41, 2.56 or similar rear gears. Just swapping that to mid 3.x ratio will give a huge increase in performance.

 

You do not need 6800 rpm or forced induction to make that level of HP from a small block. However you will need to spend way more money than it will cost to build a modestly modified 455.