Back in 1970, I sometimes rode to work with a country boy with a yellow 39 Ford coupe who lived near our house full of teenage hoodlums ( Gatlinburg summer jobs ). I started to say he shifted that car like somebody's grandmother, but I think most grandmothers were harder on transmissions than him. I asked him one day why he drove like that, and he told me many broken trans stories. He had a 283 in front of the stock box. Lonnie, are you out there? Tell us more stories.
You might not like that early Chev 3-speed either behind that 400 SB. You might be better off with a later Saginaw 3-speed.