Load-levelers do what the name implies: coupled with a coil-over outer spring, they greatly increase the carrying capacity. Today, they can even be had in gas-charged versions. I used these on a '68 Chrysler 300 parts-chaser I owned, and had a set on a '67 GTX in which I carried a lot of tools, and they made a real difference. They will not make any major difference one way or another on the dragstrip.

Air shocks are a combination of air spring and shock absorber, and this is what is on the R/T right now (they have been under the car since the '80s when raised back ends and white air shocks were the "deal"). Air shocks allow you to adjust the shock's performance based on the load; when I am driving to the track with slicks and test parts in the trunk, I can pump them up to 50 pounds (they'll take 150 psi) to maintain ride height and keep the pinion snubber an inch off the floorpan. At the track, I run 20 psi in the passenger side (right rear) and 10 psi in the driver's side (left rear), which, when coupled with the Super Stock springs, keeps the snubber 1/4 inch off the floor pan. Route the air lines away from exhaust components for obvious reasons, and use two separate inlets (one for each side) so you can accurately judge and adjust pressure levels.

Suspension science is not difficult; the factory did a lot to make getting a grip with your Mopar an easy task. Decide what you will do with the car and buy parts according to the overall need of your package. Street/strip cars will indeed benefit from upgrades and doing it right will keep you from having any "shocking" experiences later!