Based on the input above regarding using the block drains and how hard it is to access them, etc. I've decided to drill and tap the thermostat housing. Aside from avoiding drilling into the thermostat itself are there any other things to watch out for? Does it matter what side of the thermostat I tap?

I just finish installing mechanical pressure gauges on my HP500's. Block drains, forget that. I tapped the intake left of the therm housing where there was an unused accommodation for a sender.

I have 1-2 psi at idle, 10-12 at 3500-4000 and 18-20 at 5500 . Anything over 30 you may start pushing water out between the head and block and intake . Here’s my 500efi at 35psi you can see where the water stains/rust was pushing out water.

https://cimg8.ibsrv.net/gimg/www.off...ac0d3d1df.jpeg

Don't have a pic do you?

https://cimg7.ibsrv.net/gimg/www.off...e1bbb3eaf3.jpg
Enlarge you will see it left of therm housing

pq what thermo housing do you have post a pic

Man what a rookiie. First time trying to upload pics on here. Anyway not real easy to see but on the left lower (below the thermostat) is where I plan on drilling and tapping.




https://cimg3.ibsrv.net/gimg/www.off...1fc60e8ceb.jpg
https://cimg5.ibsrv.net/gimg/www.off...18229c6883.jpg
https://cimg7.ibsrv.net/gimg/www.off...160cb53143.jpg

sorry I didnt see your first post being 500 efi, that should work