Do 2000 carb HP500's need thermostats?

Yes. Keeps temps and power consistent.

If they are fuel injected-------YES! The ECM takes temp readings and adjust the fuel mix.

Dail

Some of the best running engines are closed cooled running at 180 degrees. It is important to bring the engine to a good operating temperature, it brings the oil up to its proper temp range, makes for better combustion which makes power, helps to eliminate condensation in the engine, and bring the engines parts into tolerance.

Mine (carb motors)do not have thermostats or circ water pumps. Just have the crossover and sea water pump. They run very cool. When I tried to put the t-stats in the pressure blew out the housing gaskets.
Is my setup wrong? I really would like to get a little temperature in these things.

You have no bypass hose then.
Do a search. This is one of the more common topics.

You will need to change the crossover and thermostat housing to the kind that have a bypass hose. Without the bypass, block water pressure will be very high and no water will go through the exhaust, resulting in burnt exhaust hoses.

Here's an interesting Bypass valve I found that is built into the t-stat housing:
http://www.rexmar.com/page160.html

After reading old posts on this subject I am still not sure of one thing. Do you really need a check valve in the system or can the pressure just be relieved by bypassing to a 3 port t-stat housing (which circulates the water back out the exhaust when t-stat is closed)?

I have a pipe plug on top of my thermostat housing. I think there is also a plug on the coolant passage of the Dart Intake.
Could'nt I replace the plugs with fittings and just run a 3/4 hose from the passage port to the top of the t-stat housing? That would bypass water past the t-stat and go out the exhaust. Would it flow enough?

CTA