If making heater core replacement in an 90 is your only goal then the dash swap is pointless because it shouldn't be any different on the GEN2 dash. Now if there is some aesthetic value to the swap as well, that is different.
The swap can be done though. Putting the newer dash in the older vehicles is commonly done with 4.0 swaps, and I have put the newer harness in the older dash. It was a lot of work and figuring, but it works really well. You will need a few plugs off the older harness and a lot of pins need to be rearranged in plugs, but almost all of them fit, so splicing is not needed or recommended in most spots. But keeping the existing dash harness is a must since the connection to the engine bay is very different.