Air helper springs?

[sleepy350]

Hey guys, I'm currently looking into picking up a boat that I plan to be towing with my 1970 C10. It is a 4 speed that is essentially stock. No power brakes but that will be the first thing I do. I been searching around for some air helper springs that can go inside the rear coils for load leveling but I am surprised that I can't find anything so far. Does anybody know of any companies that make a helper spring for our trucks? Or maybe an application that will work with our springs?

Thanks,
Angelo