Which valve springs for my 4.6

The first set.

By "power adders" they mean blowers which pressurize the intake. If you have an intake valve that's 1.75" in diameter (e.g. 44.5mm), that valve has an area of ~2.4 square inches. With the intake tract pressurized to 10psi, that means the pressure in the intake is producing a force on the intake valve of 24 pounds. And that's just 10psi...if you're pushing 20psi, now you've got 48 pounds trying to push the valve open.

You can see how even a set of standard Comp "beehive" springs may not offer sufficient seat-pressure (93-pounds) under boost (93-48 == 45 pounds) to ensure good sealing. They offer a "high load" set giving 120 pounds on the seat and 275 pounds open for this reason.

For engines revving in the normal range and not experiencing large boost, there's no need for high-force springs. And remember, spring pressure is not the only determining factor for valve stability at high-RPM...

Springs can be done with the heads installed.

If you don't have access to air you might try this:

Obtain a long (several feet) length of nylon rope of, say, 1/4" so (thin enought to fit through the plug hole...) Set the cylinder about to be worked on at mid-stroke on compression (both valves closed, piston on the way up.) Feed a foot or two of rope into the cylinder through the plug hole, leaving plenty hanging out. Now, using hand tools only, turn the crank slowly to bring the piston toward TDC. Stop when you feel resistance. The rope is bunched up between the piston and the valves and valves won't go anywhere as you work on them.

When you're done, turn the crank back just a bit, enough to relax the pressure on the rope, and pull the rope out of the cylinder through the plug hole.