I have searched and searched through the forum, found many posts, with many different answers.

What is the best recommended spring for the HP500 carb motor?

Stuck between:

Isky - 8005-SP
Isky - 8205 Plus

Which is preferred and why?

Also, if valves are needed, what do you guys recommend for valves?

Both are good, the plus is a endurance spring. I chose the endurance spring, it should last longer. A little more money but more durability.

What Cam the stock HP500? The Isky 8005 are my choice. Never had a set fail yet, as long as pressures are set right. And you are using good valves. My 2 cents.

Yes, stock cam.