Check engine light ls swap

[tow21]

Any help would be great, so I just finished a ls swap on my wife's 70. I've replaced everything new waterpump, thermostat, hoses, temp sensor. The problem is it will kick on the check engine light saying it's a p1258 code (coolant temp sensor). The truck never goes over 200 mostly stays at 180 could it still be air pockets? I've ran it with the cap off and watched the flow and air bubbles come out but still problem exist really starting to get aggregated with it