You seem to have a non-standard setup (Not described by repair manuals for standard models). I think I see a carbureator instead of a TBI or multi-point fuel injection, the air cleaner is modified from standard, and you mentioned a performance manifold.

You are prety much on your own for finding specifications. However, there are guidelines you can use. Look up the opening temperature of thermostats for engines of the era for this one, probably the 1970s. While you are looking, also record the pressure setting for the radiator cap. The cap's blow-off pressure goes up with coolant temperature.

Recalling from memory (not accurate), thermostat opening temperatures were increased in the 1980s for clean running, etc. I seem to remember seeing a number of thermostats at 195 F. If you lived in a hot climate and the engine ran a litle too hot, the thermostat could be lowered to 190 or 185 F. Even on those early sytems from the '70s and '80s, the engine had to attain a high enough temperatuere for the emmissions controls to work. For example, I had a 1980 Cevy Citation and it ran in the start-up mode until the coolant passed 140 F. Some of the ignition advance was held off until passing 140 F. This was done to speed up warmup on cold mornings. Holding back on ignition advance made the engine less efficient, and less powerful, and made it heat faster because the driver gave it more throttle to do the same job it would have done with usual settings.

A 180 F thermostat is likely about right for your setup. I would not go above 195 F. Here's what to do. (1) Look up in GM repair manuals for thermostat temperatures for the years that represent your setup. (Chilton's manuals are useless in the years of 80s and 90s. Those do not specify thermostat temperatures.) (2) Go to an auto parts store and look up thermostats for the year of your engine. (3) Search the internet.

With the thermostat closed, there should be a passage for coolant to flow through the water pump and the head and possibly the cabin heater core. This reduces cavitation in the water pump, evens head temperature while warming up, and allows the thermostat to sense the rising coolant temperature. It also provides early cabin heat, although using the heater slows warmup.

By the way, the top of your engine is clean.

The Chevy small block is in a streetrod. The temp gauge takes forever to move and usually stays at 150 f. I was somewhat surprised to see the gauge hit 200 during 100mile trip. Speed was 65mph for most of trip so I had good airflow. Am driving back today and will monitor temp. But it's cool. Perhaps 60f today.

I'd put that temp gage in a pan of boiling 212úF water see how it responds - fast or slow. Be a bit tricky but doable. Have to hook a ground jumper to it.

I had a '70 Chev small block with auto transmission that wouldn't curb idle in neutral no matter what I tried. In 1982 (100K miles) the thermostat stuck open. It was 165úF thermostat from the factory! Correct was about 195úF as I recall. After that it would curb idle. Seems incredible but that's how it was.

Don't know what 195AF is. That wasn't me it's this editing program. :-)

There are two types of coolant sensors used on automotive engines. One toggles closed at a set point of maybe 200 F or more, and then opens at a temperature below the set point; while the second type chamges resistance smoothly with temperature change.

The first type (toggles quickly) signals the engine controller to turn on the electric cooling fan.

The second type (analog) supplies an analog signal to the engine contoller and in turn operates the temperature gage. In some cases, this sensor may be directly connected to the temperature gage.

sensor is tied directly to dial gauge. stayed at 150f today. got in construction zone for a bit and it went down. 130f or so? decided to change thermostat. turns out it was 180f. let car idle and it got around 180-190f. drove home and it was back to usual. 150f.

My oil pressure is 60+ at 55mph. Barely drops at idle. When motor hit 200f last week I saw oil pressure hit 25psi at idle. Now that motor is cold like usual, it stays at 50-60 at idle. I think motor is too cold for most part. Or maybe I look at gauges too much?

Where is the sensor located for the temperature gauge? It should be screwed into the head or thermostat housing, and it should be near the thermostat. If the sensor is sitting on a bracket, it may not accurately register the temperature of the hottest coolant. Air flow around a bracket will cool it to below engine temperature. Air flow can be a mix of fan induced flow plus road speed induced flow.