Heat gauge redlines during first cycle

Recently purchased used truck 94 c/k 1500 350 with lift kit, they rebuilt an engine and put in it. The first time I start it in the morning the heat gauge doesn't move and then slowly goes to the redline border, stays there for a minute or less and then goes back down to normal. The oil pressure gauge also flickers and goes pretty low during idle. The carlot said the head gaskets had been replaced but not the cylinder heads. I just replaced the thermostat. Any suggestions...

Is the Temp guage lying to you?

First you need to verify that the motor is actually getting really hot.

I'm thinking you may have a bad Temp sensor, or else excessive resistance in the temp sensor circuit.

Are you losing coolant anywhere?

When you initially start your truck, do the radiator hoses get hard immediatly?

The coolant is allways plumb full to the top. How soon are the hoses supposed to get hot? I'm inexperianced but no they did not get hard immediately (at least a few minutes)and then they felt pretty hard especially the hose that takes fluid from the radiator to the engine.Is the Temp guage lying to you?

First you need to verify that the motor is actually getting really hot.

I'm thinking you may have a bad Temp sensor, or else excessive resistance in the temp sensor circuit.

Are you losing coolant anywhere?

When you initially start your truck, do the radiator hoses get hard immediatly?

If the hoses stay cool on initial start up, but get very hard, that would lead me to believe it has a bad headgasket.