We have a restored '38 Canopy Express and I think the transmissions may be close to the same. I think you probably still have a 206 and mine is a 216. But to answer your question, let it soak until you are satisfied those seals have swelled up enough to do the job of holding the usual transmission fluid and will not run past the seals. For any more I'd try either Larrowe and Sons in Big Sandy, OR or the Filling Station in Lebanon, OR. Both have some good knowledge about this vintage of Chevrolet's.
Normbc9
