They were pretty common in early to mid 60's GMC trucks & busses. The smaller ones in pickups were 305 cid and the bus models were 351cid and bigger. Yep they are huge and heavy and hold 6-8 gallons of coolant.
I rebuilt one for a customer about 6 years ago. There must be an inch of deck between the cyls, 4 ring pistons, really long con rods, and they redline at around 3600 rpm.....built like an industrial engine and make a bunch of torque compared to a 305 v8.
Boat anchor? In the real world, yes, but kinda cool just the same......
Johnny