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My recently rebuilt M 38 engine runs about 190 degrees when driven at 45-50 mph. it has 160 degree thermostant . Is this to hot?

Yes if your gage is accurate. Remove your electric sender from the head and with the wire re-attached and a ground jumper from the sender body to the block hang the sender in a pan full of water (don't let it touch the sides or bottom of the pan) and using a propane torch bring the water to a boil. With the main switch on the temp gage should read 212 degrees with the water boiling. If your gage reads 212 then you have a problem with your cooling system. If, for example your gage read 25 degrees higher than 212 when the water boiled then that would mean your actual coolant temp was 165. That means you have a problem with your indicating system. The troubleshooting tech bulletin on Bob W's web site http://www.cj3a.info/sibling/milgauge/milgage.htm