I just got the car started tonight and was warming it up to set the timing and it went to 250 deg in a big hurry. I've been doing some ignition work and engine bay cleaning . I also noticed the water pump belt was loose so I put on a new slightly smaller one about a 31 1/8. This car always ran nice a cool around 180 deg.

Any suggestions

 

 Nice to hear it's running again. 

  Three questions,

         Did you take any hoses off ? If so, you might have a bit of air in the system and that causes the temp to spike. Mine did.

         Did you bang around on the Water Temp Sensor on the side of  the block? It's pretty sensitive.

          Have you checked the thermostat to see if it's still good ?

           The water pump belt being loose might cause it but if you tightened it up, it's time to keep looking.

   Jim

You may also have failed thermostat.

what all did you do to the engine ??

if this is the first start after a rebuild then it will run hot regardless