This thread got me thinking, and sure enough, between all the switching between my Suburban and Stepside, I had the TBI water pump and no bypass hole in the heads. I nabbed a water pump at Pull-A-Part and switched it out this weekend. Here are some pics of the differences.
The water pump on the left is the older-style internal bypass water pump. On the right is the Vortec non-bypass water pump.
View media item 32317Note the two tubes at the top. The one on the left goes to your heater core. The one on the right goes to the heater hose outlet on your intake manifold.
There's also some discussion about Vortec water pumps on TBI blocks, and this pic shows why. The old style pump on the left has a coolant port under the bolt hole on the passenger side, while the Vortec on the right does not. The port in the block is between sizes for a standard bolt thread, so I tapped mine metric and found a bolt that would thread in it. I cut the head off the bolt, put a slot in it for a flathead screwdriver, put some RTV on it, and threaded it in until it was flush.
View media item 32318