Overheating V6 vortec

I replaced an engine in a 1992 Jimmy and the rebuilt long block I received
was from a 1995. I replaced the radiator, all hoses, heater core, and
the water pump. I was told by a mechanic that it is overheating.
He said on side is running at 210 deg and the other is running at 300 deg. I guess one head gasked could be on backwards, although
I really don't know. The intake both uper and lower are from the 92.
Any ideas? Manifold different?

Check if the head is on properly.

Sounds like air in the cooling system. Did you do a flush and fill on the new block?

Install a new non AC Delco Rad cap as well.

Is there a special way to fill the cooling system on these vehicles?

Can a head gasket on a vortec v6 be put on backwards?

Thanks for the reply BlazrerLT

Best way to getting the cooling system going is to run the system without the thermostat so that the engine will be able to prime itself without the thermostat holding the coolan back.

Keep the rad cap off and monitor it and fill the rad when the coolant starts to drop.

It will take a while to burp the baby.