You need to keep the G60 cluster, as as said its got a special vac unit on the back to cope with boost.
Easiest way round the problem is to buy another set of MK2 clocks, and swap the speedo into the G60 cluster. Much easier than stripping the speedo down to swap another face on it, and it doesn't matter what max speed speedo you get it'll read just fine

Early/late speedos will all work, but the mounting on the back for the voltage regulator is different so ideally you want one that matches your cluster.