Ok, here's my overly long and rambly video that shows the back of the PRW water pump with the early SBC bypass holes; and the back of the Stewart Stage2 that does NOT have the bypass holes.
It also shows the difference in the rear covers and fasteners:
https://www.facebook.com/776771928/videos/a.10154956065381929/869456011552959
(The video file was too large to directly upload here, so a link to FB will have to do.)
These are the button head stainless bolts that I used on the rear cover of the Stewart Stage2 Corvette pump so that it would clear a thick aluminum timing chain cover and seat and seal properly:
This picture is I think stolen from somewhere else on this site, and shows the hole that I drilled slightly larger and tapped and installed a 1/2" NPS Fine thread plug in. (I accidentally bought an NPS tap instead of an NPT tap).
I installed one of these plugs into the block hole and added plenty of Permatex Thread Sealant on it:
I installed a Felpro ES 73030 blue paper water pump gasket onto the water pump after cutting off the unnecessary 3rd hole and attached it to the water pump with Permatex 22071 Water Pump and Thermostat RTV Silicone and let it dry for a few days, then installed a bit more on the block surface and installed the water pump.
Reminder to anyone reading this: Performance cooling applications get rid of the stock bypass hole to attempt to even out the temperatures between cylinders (a modern performance head gasket, and getting rid of the bypass helps, as does installing rear-of-intake to a front-of-intake coolant lines, but that's another subject). BUT,
you always want to maintain some sort of coolant bypass that allows coolant to circulate through the engine BEFORE the thermostat opens.
A late SBC Vortec block accomplished this with a small tube going from the front of the intake to the top of the water pump -thats a good way to do it and it will allow a lot of flow to recirculate BUT, some people think it looks bad and it's definitely one more leak path and one small, strange length of hose that may need replaced.
An alternative is to use one of the EMP/ Stewart SBC high flow thermostats that has 3 holes drilled in it. This approach will allow LESS coolant to flow vs. the vortec or big block style bypass hose until the thermostat opens and it will also cause the engine to take a bit longer to reach thermostat open temps, BUT, IMHO, it has some additional benefits worth considering:
1. It allows the coolant in the radiator to slowly come up to temp which introduces less thermal shock to the radiator; thermal shock of the radiator going from having only cold coolant in it to have a huge "slug" of 180F engine coolant rush into it, can start to break radiator end tank welds over time (this is a reason that some aluminum manufacturers that weld their end tanks to the end of the tubes still use epoxy (See:Griffin Radiator's explanation on their site) and it's a reason that many OEMs require both welding and some epoxy to help protect those seams; the epoxy also protects the welds from vibration-related damage in things like high-speed off-road racing apps.
2. the increase in temperature of the coolant in the radiator allows the pressure of the cooling system to increase more before the thermostat opens, which helps reduce localized boiling
3. It may prevent / reduce initial "bouncing" of temperature right after the thermostat opens. The early SBC setup usually sees the coolant in the engine heat up until the thermostat starts to open, then a big "slug" of hot coolant rushes into the radiator and a big "slug" of cold coolant rushes from the radiator into the engine and then hits the thermostat and can cause an overly fast-acting thermostat to then slam back closed again and temperatures can bounce pretty wildly as the engine warms up. (These swings are hidden by slow acting mechanical gauges, but can be seen with fast-acting digital gauges are are normally attributed to "Chinese digital gauges being junk" -when they're simply showing what's actually happening, but is normally hidden by a slow-responding mechanical gauge.)
4. Having a couple of small holes in an SBC thermostat is already a good idea to help purge air from the system, if you're not filling a system that you've pulled a vacuum on.
-All these are reasons that GM LS engines moved the thermostat to the water pump INLET side of the cooling system vs. the intake outlet side. Blocking off the SBC bypass port in the passenger side of the block and running a thermostat with bypass holes drilled in it, makes an SBC adopt many aspects of a late SBC Vortec cooling system AND the later LS cooling system setup.
P.S. The EMP-Stewart thermostats are simply rebranded high-flow Robert Shaw Thermostats with the holes drilled by EMP-Stewart, you can usually buy a Robert Shaw thermostat OR a "Mr Gasket" high flow thermostat, which is also just a rebranded Robert Shaw and drill a number and size of holes of your own choosing. This style thermostat is recommended or REQUIRED for MANY SBC high-flow water pump companies such as Stewart and FlowKooler.
The Mr Gasket 4364 on Amazon is currently the cheapest version with no holes pre-drilled. Speedway Motors has the cheapest Robert Shaw-style thermostat WITH pre-drilled holes. The EMP Stewart 301 (180F) or 302 (195F) is pricier, but is all copper / brass.