Impossible to install spark plugs!

Wow thank you so much everyone from the abody community. If it’s as easy as putting a smaller plug in that would be awesome. I know the mechanic that previously changed my plugs years back put 13/16” AC delco’s in the old iron heads. The car hasn't run in two years so I wasn’t sure if that size was the correct plugs. Am I fine using the RC12YC champions and what is the approx set gab for the Edelbrock’ s? Would you just say .030 or .035? Thanks again everyone. I’ll keep you all updated.