That SUX!
I remember doing my 69 Bee 4 speed Hurst Ramrod with a chain and it stretched like limp spaghetti. I Put a solid 3/16 x 1" strap on and it never failed again. It was shakey but I was young and it was all good to keep the motor in place! Try this bolt solution for your 383S and only tighten enough to give a little less than tight compression on the mount. Install the bolt from the bottom and nylock nut on top. It will hold and not shake you teeth out if you have a big cam in it.