From the drain plug?

It's not the petcock but there is a plug on the lower half of the block

I have this same kit on my 502 mag mpi, the crossover w/bypass and thermostat. I doubt you will need to relieve pressure, and actually I am having the opposite of a pressure issue. From an experiment we did, it looks as the bypass flows to much water once the block fills up and provides resistance. We put a 1" OD 3/8" washer inside the hose pressed against the tstat housing and it increased the pressure in the block. Before I wasn't reading anything on my gauge and when we did that it was too much pressure, we are thinking that with the resistance in the block the water just takes path of least resistance and flows through the bypass and right out of the exhaust. My guy said he had a buddy with a fountain that had the crossover with bypass and thermostat that had a overheating issue and this fixed it as well. I would think this would be a known issue? Or is this not common? Most people from my understanding run crossover with no thermostat or bypass, so don't really know how many are running the bypass with thermostat to even experience this. I am running my gauge from a block drain as well, don't have anywhere else to put it, as the tstat housing in this kit doesn't have ports for that, and my water temp gauge is ran from the port on the front of the intake.

Previously installed cross over did not have a thermostat instead was running a restrictor plate which caused 2 issues
#1 No water temp (Less than 110 at WOT)
#2 No water pressure in the blockhttp://i129.photobucket.com/albums/p...ps0ef7b0a2.jpg
http://i129.photobucket.com/albums/p...ps5be7bde4.jpg

Installed new cross over and now running a Thermostat and added a pressure gauge. Installed 7' of 2" thin walled PVC pipe under the gunwale from the the engine compartment to the dash then was able to run the pressure line through the PVC.
Located the engine block drain on the port side below the heads.
http://i129.photobucket.com/albums/p...ps0e624b53.jpg
http://i129.photobucket.com/albums/p...ps3c70331d.jpg
http://i129.photobucket.com/albums/p...ps1de2bce8.jpg
http://i129.photobucket.com/albums/p...psf99a2872.jpg

Forgot to mention that I added 3 T's to the PVC for future wire runs in various locations

The hardin marine one that I am running has a third outlet on the thermo that goes down to the crossover itself that allows water to flow from the thermostat to the crossover before the thermostat opens...