First off, these crossover holes are there to heat up the bottom of the carb plenum for better and faster engine heat-up in very cold Chicago winters.
If you close off this port, it affects the heat riser valve, causing it to open before it should.
For us'uns that only run our classic cars in good weather or in warm climates, we really don't need a heat riser valve (the 430 has none). I gutted mine with a cutting torch but left the outside looking like it's still intact.
I also used shim stock to block off my crossover ports. I didn't do it to keep paint looking pretty, I made this change to get rid of heat and make it exit the exhaust manifolds asap. That way, the cooling system doesn't deal with it.
BTW, I get my Y-block gaskets from Rockauto.com. - Dave