spark plug threads

Hi all, I recently replaced the cylinder head on my 1969 4HP (#4w69b) because the threads were completely stripped on the upper cylinder hole. The new(er) head (from a 74) went on great, although the engine didn't run that well. I've determined that again on the upper cylinder hole, there is air leaking past the spark plug. The threads are in great shape, although there is a gouge where the seating surface is. I bought a SAV-A-THREAD kit in hopes of installing an insert, but the angle of the spark plug holes won't allow the tap to go all the way through and thus the larger tapped hole can't be cut. Are there any other options out there for fixing this problem? I've heard that the regular heli coil spring inserts don't work well for this application. Would a high temp sealant on the current threads work? Thanks for all the help.

heli coils do work fine for spark plug holes.

Are you refering to the regular spring-like heli coils or the solid inserts? Wouldn't the tang pose a problem? All that I can see available for the m14 1.25 heli coils are the inserts, not the spring like ones...

the tang is broken off, you remove the head to do the job.

The problem is the regular spring like heli coil is not made in a M14 1.25 configuration. It comes in 1.5 or 2 threads per mm.