Ted,

Even with a Davis crank, try a 7-3 or 7-4 first. You may get enough RPM, at softer compression and a bit richer needle... thus less fiddly to work with...

If all is well, and you need a bit more - THEN try a 6-4 (or 6-5 if you find one). The only thing that annoyed me about the DDD .049 conversions was that a bit too hot, or overcompressed, might burn through the head seal - needing replacement before the next run. That was years ago, Bob D may have changed things since...