Might sound a silly question, but how do you know it's leaking? And as Russell asked, is it definitely that join and not a couple of fins lower on the pulse pipe side?
Rather than lap the faces together, if it is definitely at that joint, I'd check them for flatness and give them a light skim if they're not within a thou. (Or get a good machine shop to do so). Might mean fitting a thicker base gasket afterwards to get the squish clearance and compressed volume right.
Also worth a look to make sure the "spigot" on the top of the barrel fits correctly into the recess in the head with all the bolt hole lining up nicely.
Liam
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