Many years ago I stripped the plug threads in a 94 Crown Vic 4.6L engine (destroyed them removing a frozen plug). Bought helicoil kit designed specifically for Ford spark plug hole repair at CarQuest. This is a common problem with these engines. Cost was minimal, about $30. Used thick grease on the ream and on the tap according to directions and vacuumed out cylinder through hole afterwards. No serious access issues with these engines. Repair seemed to work OK. I Drove the car for a few months before selling it. 
99 Tropic Green SVT, Tan Leather, 20K miles, "Nice Twin" (factory stock).
99 Tropic Green SVT, Tan Leather, 28K miles, "Evil Twin" (Turbo AER 3L and more in progress)
96 Red LX, Opal Grey Leather 2.5L, ATX, 22K miles
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