Anthony, Going on the assumption (which may not be correct) that your heads were modified to fit some particular type of spark plug in the past, I think it may be more productive to try to figure out what that plug was than to further modify your heads. Any modification would probably reduce the number of threads holding the plug, which is not a good thing with aluminum heads. In one of my posts I posted a link to a site that has the parameters for most brands of plugs, including reach. I think that some time going through those tables to see if there is something out there that may fit the heads as they are now would be more productive than cutting the heads again.
Here is that link again :
https://www.sparkplugs.com/learning-...bering-systems.
I find the Champion table to be the most comprehensive and usually use it. If you find what you want, but don't like Champions, you can always find another brand that will cross-reference to the Champion plug.