Differential Fill Plug Where Is It?

I recently purchased a 1976 Dart Sport with a 318 and 4 speed manual transmission. Differential is leaking fluid on passenger side where the axle tube enters the differential. Will try to stop leak with JB Weld SteelStick epoxy putty. Need to refill differential but I don't see any fill plug on the diff cover. There is not much space between diff cover and gas tank. Felt top of diff cover and there does not seem to be a fill plug on the diff cover??