Thru a couple of my threads I've posted about using a 60 cover on a 66 389 and the flat area of the front of the block sealed up with just using the 60 gasket and no changes were needed to the block. There was a change not sure when about the front of the block not being flat all the way up to seal the unused holes. This hole on the cover was pretty rough and would need machined to be able to seal up with a freeze plug. I tried with just sealer on the freeze plug and it didn't hold.
What year block are you trying to use?
There are some pics of this on my Hurst mount thread .
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Illinois Outlaw Gassers
6.27@107
9.97@131
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