Industrial 318 head and valve seal question

I've got a 70s W300 with what appears to be a 318 with the industrial or truck heads or whatever you want to call them. It takes the big 18 mm spark plugs. The only casting number I can find ends in 227-1 and I can't find any info on that number. It also has "TRUCK" cast in the head also. Seems it needs valve seals so I was wondering if anybody could give me a part number for the valve seals. Preshate it.