Any piston/valve contact is too much. The fact that the valve is stuck in the guide probably means the stem is bent. Replace it. Stovebolt intakes are virtually vertical in relation to the piston, but don't rely on that to mean the piston will simply punch the valve straight back up into the guide.
On the carbon fouling- - - - -you've got one carburetor too many. Dual carbs on a stovebolt are eye candy. Unless you've done some extensive modifications to make the engine need that much carburetion, and you only plan to run wide open throttle most of the time, trash the dual carb setup, buy a 3-carb manifold, and make the two outboard carbs non-functional dummies with a solid plate between the carbs and the manifold. You'll get much better performance and the ignorant mouth-breathers at car shows will drool all over your fancy engine compartment!
The only 2 barrel I'd consider would be a small-bore Rochester for a 265 V8, with enough modification of a stock intake manifold to make it fit, and flow enough volume to accommodate the extra capacity. I'm working on that project, boring the hole under the carb out about 1/4" larger, and fabricating an aluminum adapter block to allow the carb to bolt on the manifold securely. The progressive-linkage 2 barrels some of the resto-ripoff people sell is a solution to a nonexistent problem unless some intake manifold modifications are done in the process of the carb conversion.
Jerry
