If the heads haven't been on for a long time, they should clean up with a rag and solvent. I too use the razor blade. Do you have a shop vac, if so, use it while you are cleaning.
I've used that method many times and it works.

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"Tell it like it is or don't tell it at all" 1964 Falcon 2dr HT, 347V8 4V, dual exhaust, 5sp, pwr assist R&P steering, AM radio, heater with defrost and 2sp fan, electric windshield wipers, cigar lighter, dlx light group, bucket seats, console and full carpet.