If the hard drive is disconnected and you already checked that the fan is working, overheating is probably not the reason why it’s in a constant restart loop. Only other thing I could think of is to make sure you didn’t over tighten the long fan screw and T6 logic board screw. if you have an adjustable torque driver, it must be set to 2.0 in. lb. Over tightening could bend the logic board, possibly damaging surface mount components. edit: I just checked and this model has a flash storage, it could be loose, reseat or temporarily disconnect to diagnose if this is causing the constant reboot.

Apple certified service technicians uses Apple netboot diagnostics called AST to serialize a newly replaced logic board. Alternately, they may boot to a blank board serializer software. I'm sure if you take it to the Apple store, they can do this for you since it would be detected that your MLB is blank and needs to be serialized when they run their diagnostics. Where did you purchase your logic board?