what should I check first?

With no thermostat a 454 block should never run any hotter than 140F and that's under load. If this is from cold startup to this kind of temp in just a couple of minutes driving you have a mechanical issue.

On most vehicles, the temperature gauge reads cold until the engine has run for a few minutes. With the thermostat stuck open, the engine can be overcooled, causing a low temperature reading. If this is the case, the thermostat may need to be replaced.

One of the first symptoms associated with a problem with the coolant temperature sensor is poor fuel economy. If the coolant temperature sensor goes bad it can send a false signal to the computer and throw off the fuel and timing calculations. This will reduce fuel economy, and may hinder engine performance.

Reasons the Temperature Gauge Reads Cold

If the temperature gauge still reads cold after the engine has warmed up, the gauge may simply be broken. ... With the thermostat stuck open, the engine can be overcooled, causing a low temperature reading. If this is the case, the thermostat may need to be replaced.

Check the thermostat gauge reading when the car is idle. If the gauge is moving from normal to cold within a few seconds when the car is moving slowly or idling, the gauge could be broken.