DISCLAIMER: JERRY'S GASKETS, IT'S OWNERS AND EMPLOYEES BEAR NO RESPONSIBILITY FOR ANY DAMAGE, PERCEIVED OR OTHERWISE, THAT MAY BE ASSOCIATED WITH THE FOLLOWING THERMOSTAT MODIFICATION. 

Drilling additional hole(s) in the thermostat can add efficiency to the LT5 cooling system.  If your cooling system has been flushed, and the coolant is fresh with the right anti-freeze mix, all the road debris has been cleared from the front of the cooling stack(Oil Cooler, AC Condenser and Radiator)........ and your LT5 is still running hot (over220f at hwy speed) then you may benefit from this modification,

Drilling hole(s) adds to the cross sectional, open area of the thermostat and (1)causes the thermostat to open quicker by coolant passing over the wax pellet chamber, especially when a thermostat is installed that does not have slots or holes in the bypass valve, and (2) reduces back-pressure on coolant flow through the radiator. 

Hole size: 1/8"

Hole location: as near to the fillet as possible, but not into the valve/valve seat

Hole quantity: Start with 1 hole. Maximum of 4.  Don't make the mistake of drilling too many holes too soon. 

The ZR-1 isn't your grandfathers 1st car.  The LT5 Engine Management Computer aka ECM, is calibrated for engine temps above 180f.  The LT5 engine was hand-built with very close tolerances designed for durability at the specified operating temps. Operating at temps below 180F can cause premature wear and damage.    

Bear in mind your climate and driving conditions. what works well in the summer may be too cool in winter.