trying to find a rear only air bag kit

[Greasey Harley]

I have a 68 Chevy short wide, with coil rear suspension. I plan on doing a set of 2" drop spindles in the front.
It would be nice to get the rear down 4 to 5 inches and still be able to haul the dirt bikes or the Harley in the back, perhaps a small trailer too.

Air bags seem like just the thing. low when empty, pumped up when hauling and slammed for loading bikes (better load angle).
I assume I would need some fairly heavy bags, as I still plan to use it like a truck.
Also, I'm not sure about drive line geometry, shocks, shock placement, panhard bar etc.
I don't want to mess up my truck and not be able to haul heavy loads.

The truck will normally ride 4-5" lower in the rear when empty, and pump the air bags up taller and stiffer with a load
best options?