I have a spacer that I can give you. It did come with the port blocked but I drilled it out. You can refill it with a heat resistant epoxy, though, as suggested. If you want a new one, I got mine from FRPP.
Do you just want to use those valve covers or are you using them because of rocker arm issues? If it's the later, may I suggest that you get TFS valve covers. They clear roller rockers.
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351W 89 Mustang GT Convertible