glow plug length

[dave gone jeepy]

as it's a sunny day I thought id finally change the glow plugs in my 1980 mitsi jeep, I had picked up some plugs from detroit diesel the engine is a 4d5a engine, the ones they gave me where bosch gpf 116, when I took the ones out of the jeep they had 3p on it.
the new ones are around 4mm longer than the old.
I know with spark plugs if that was the case not to start the engine in case the piston hits the plug, in a diesel does the glow plug sit over the top of the head or is it more in the inlet manifold ?
The manual I bought didn't help much with that lol..