what is Inst1?
are you sure you connected all the same coloured wires at the body harness connector when you did the SOHC to DOHC swap? one of those is the tach wire so if you didn’t connect it then your tach won’t work.
the tach signal should come from the ignition coil.
if you’re sure that everything is in order then pull out your cluster and try to find the tach wire behind it, it will probably be the same colour as it is by the ECU. measure this wire and make sure it has the correct signal. if it does then your cluster is probably the problem.
hope that helps.