You have a more serious issue than just a thermostat if it is getting that hot that fast. This is an 85 with a 350 in place of the stock 305 correct?
Which parts did you use off which engines? I ask because it's possible you have the wrong water pump. GM used two different pumps for a few years 1985 was one of them. If the car came with conventional V belts it had a standard rotation pump. If it had a serpentine belt the impeller turned the other way because it drives off the back of the belt. The fan rotates the opposite direction as well. Both of those would cause the engine to run hot.
The other items that will cause a hot engine are advanced ignition timing, high compression, lean fuel mix, plugged/restricted exhaust or a blown head gasket.
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Saturday, August 26th, 2017 AT 6:27 PM