Spark plug size help

[Sonny]

I am currently working on my 1978 360 LA engine to put in my Belvedere (Karry Van engine). I cannot seem to find the right spark plugs for the engine. A normal spark plug for my 318 does not go deep enough into the block to catch the threads. The base hits the seat. I've been to AutoZone and Napa and they keep giving me the exact same spark plug. Can someone tell me what plugs this engine uses?

 

[aeon280]

I just changed mine and went with rc12yc champion plugs. These are i believe the magnum 360 part # but are the same as the LA 360 plug just with the 5/8 collar instead of the 3/4 bolt collar.