The "original" shocks were not air shocks and thus did not require any air. Sounds like you have some aftermarket stuff on there which was very common back in the 70s/80s. Its not about the PSI it about the ride height, so put enough air in them to get the stance you want or enough to clear the tires.
70 Challenger RT 440-6 EFI, 73 Cuda 416-6 EFI
05 Hemi Durango, 01 Ram 4x4, 14 Ram 2500 4X4, 10 PCP Challenger 6 spd RT, 01 Viper GTS ACR, 52 B3B w/330 Desoto Hemi, 70 Hemi RR (under const)
9 x Challengers. AAR Cuda, 4 RR, 2 GTX, 4 Chargers, etc... (too many to list)