slant six and big block have external oil pumps and the pressure relief valves are accessable...but no dont mess with them there will be a pressure switch (electrical ThInGie) with one tan wire leading to it...unscrew that and hook mechanical gauge in it's place
I think you`re confusing your car with your water heater. No pressure relief valve here. If you`re unsure of what you`re looking at then get someone who knows their way around an engine.