1990 3.1 V-6 over heating

The first engine overheated found problem with block cracked. I put in a low mileage used engine ran great for 2 years and now it's doing the same thing it runs good and cool with no thermostat. but install themostat it pegs the guage and overheats. Was there a problem with the 3.1 ? Is there a better engine swap? I may consider a 2.8. But I don't want a major change over like the 3400. Any help with this would be greatly appreciated. Thanks

your radiator could probably need to be flushed. If that isn't the problem get a lower temperature thermostat. The 3.1 is a good engine (reliably wise) I have two that have gone over 160000 with no major repairs just scheduled maintenance. I would not go with a 2.8 over the 3.1. Just check all the water hoses for kinks and flush your radiator.

Please tell me you're not using an old thermostat to check this out.