Thanks guys!

When I was testing fitting the engine, the piston was definitely running closer to the crosshead guide end, so perhaps a bit more gasket material on that side will give things a bit more breathing room.
Hey Rog, I've been visiting the indispensable Temple of Steam for years now. I doubt there's a Jensen in my small collection that hasn't benefited from one of the tips mentioned in the Restorer's Toolbox. Thanks for the info!