It's definitely a 6 cylinder. Here's a link to a variation on the same theme.

http://www.stoveboltengineco.com/tomsride.htm

I have seen another picture of the other side of it. There are six plugs. The motor family includes 228, 248, 256, 270and 302 cu in 6 cylinders. They are GMC Truck engines used from '52-'59