Hello, So I am looking to fire up my new build. I went and got Plugs and realized my 71 Valiant is recommending a plug much longer than what I pulled out. I do not have spark plug tubes, as I think they were OE until 1976. Of course I can not read any casting numbers. Would it be safe to assume that at some point in this cars history, a different head was installed? I shaved 40/1000 off the block and 100/1000 off the head. I am afraid to put the longer threaded plugs in and punch a hole through all my pistons.

Any thoughts???

If correct, then someone has swapped to a 1975+ head or engine...

These use a tapered seat spark plug, correct spark plugs would be NGK UR-5, AC Delco R-45TS, etc... (tell the auto parts store clerk you have a 1976 Plymouth Valiant... or for better laughs tell him you have a 1978 Chevy Nova with a 350 and you only need 6 plugs...LOL).
Oddly, you have only cut a mild amount out of the block and head and the strap and electrode on the early style of plug probably will not hit the piston, but... they won't seal correctly since they seal at the flat shoulder, not at the taper at the top of the threads... once you get the correct set of spark plugs you will see what is going on.

thanks.