I think you're right, so long as it doesn't hit anything (too long) then they will run perfectly well on almost anything.

Its surprising the J8 isn't listed in the CMP manual but these old Champion designations are now almost one type anyway.

I have to say over the years we've had continual success with NGK plugs in all manner of old stuff.

Of course it helps the Ford to ditch that iffy original coil with its ballast resistor and just use a modern 6V one; in fact the original can stay in place for show with dummy wiring too.

Here's a couple of handy references:
http://www.spark-plugs.co.uk/pages/t...ymbol_code.htm

and

http://www.spark-plugs.co.uk/pages/t...on860-n288.htm

R.