I'm not so sure a 'downdraft' carb makes that much difference. They were very popular long ago for the reason you give but that may have been more marketing than engineering. There's more to inlet manifold design than just a pipe connecting the carb to the engine and the diameter, length and surface finish all have a role to play in mixture control and gas flow. It's very easy to guess these things and you might get lucky, but having gone to all the trouble of the bore and head work you might want to approach it more scientifically. There's plenty of books that'll point you in the right direction - A. Graham Bell's Four Stroke Performance Tuning for example is one I have. Or you could ask on an appropriate forum on Facebook, but there, I suspect, if you get ten replies you'll have twenty opinions.
The only way you're really going to know what works and what doesn't is to run before and after checks on a dyno. Much quicker and more accurate (although more expensive) than just running it up the road.
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