Originally Answered: One spark plug is longer than the other?
That one guy must not understand the question.. It's not cause one brand is diff like he said.
You are talking about the threads or the reach of the plug. He is talking about the outside overall lenght.
Now the reach of the plug depends on the year of the heads thats on your engine. You didn't say the year..
but here is a gen break down for all gen I small block chevy engines.
1955-1967 used 14mm threads, .375" reach, washer gasket seat, 13/16" hex
68-95 used 14mm threads, .460" reach, tapered seat, 5/8" hex
96-02 vortec used 14mm threads, .708" reach, tapered seat, 5/8" hex
And just for the heck of it I throw in aftermarket heads like Brodix, Edelbrock, ARF alum heads
They use a 14mm thread, .750" reach, washer gasket seat, 5/8" hex
If you install a plug with a reach thats too short the firing tip of the plug will not stick out of the hole far enough and will be harder to light the air/fuel mixture.
Install a plug with too long of a reach and it could hit the piston or valves and cause major damage.
In your case sounds like wrong plugs installed