I've had Dynaflow Buicks since I was 17, and that was a loooooooonnnnngggg time ago. Still have one, along with many other makes. These are superlative cars, and I've owned most American and some foreign makes and still do. That 364 V-8 in your '57 is a great engine ... swapping a 401 won't gain you much. However, doing a bit of massaging of the 364 while it's apart - say swapping a few higher-vitamin Century or Roadmaster 364 engine bits - will wake it up! Regarding the Dynaflow and leaks - I think good ol' Transmission Bob here in Tucson ("Same Old Bob, Same Old Place - Since 1968) says it best: "Drive a Dynaflow every other day and it won't leak. Drive it once a week or once a month and plan to have a drip pan under it when parked." I would wager that trans in your car will need nothing more than fresh fluid in the trans, and don't forget to drain the torque converter too. The AACA website has directions for this under the thread "Dynaflow Question." Good luck and I appreciate your plan to keep it stock!