The Passon 5th-gear OD is 0.70.
I'm doing the same thing by using a built 200-4R automatic with a 0.67 4th-gear OD. No cutting of my trans tunnel either - and going to run a 4.30 gear as well.
I know exactly what you're saying. Years ago one the young morons broke 3 wheel lugs in a brake drum on my '69 340 Swinger because he didn't know about left-hand threads on Mopars. He didn't seem to get it even after I explained it to him.