Tough and muncie 318 don't belong in the same sentence. Back in the day (mid sixties) anyone with one of them carried a spare tranny in the trunk. Thankfully only a 45 minute swap in a parking lot somewhere.
Hurst makes a 3 speed Mystery shifter for them. Or there are some gm shifters also available. Search for 3 speed shifter on ebay.
And Welcome to the forum!!