The GMC 270 has a bypass water pump/tstat housing, but the steam tube is from the rear of the block (around cyl #5) and plugs into the water pump via fittings.
It might be worth looking at to see if they are close. But I know the t-stat in the 270 is a simple single action.
One steam relief method used on 235s in the 40s (well, the G506) was to pipe it directly back to the radiator. The water pump will circulate water through the relief tube.
Not an original set up for your truck, though.
I'm not to concerned about it looking original as long as it looks vintage. I will research it.
Thanks
