FL 70 suspension conversion

aftermarket, there are 2 diff sizes of super singles.
445/50/22.5 and 385/65/22.5 if you have the 10 lug pattern will be a Brease to swap them out

 

OP, the air ride conversion on a FL70 is worth it. I have personally had this done to my old FL over a decade ago.
The ride on air is FAR superior in any weight or empty to springs on these trucks. The spring suspension is god awful rough, I know how you feel.

I also had an "Accumulator" tank added between the bags and the leveling valve, it also made a huge difference in ride quality....would highly recommend one with what you're wanting to haul.

I believe I had around 4-5K in everything to do the conversion and it was worth every penny.

 

Aftermarket, couple of manufacturers, Michelin is the most popular.
check salvage yards sometimes wrecks come in with them on and the savings can be huge.
Good luck with the changes, I got a soft spot for those Freightliner FL series, lots of parts available for them, and maintenance/repairs are easy and affordable. Treat them right and you'll get good value for your money.