I have a HQ tonner that we cut out dash frame to put HZ dash in.(15 or so years ago) The way I decided to do it was to cut across dash top where the windscreen sits, so not to see weld, and HQ is different as it has a strip/channel for the dash top to hold in place. It wasn't a small job. But now that you can buy replacement plenums, you could just buy one for the model dash you wanted to fit, remembering that the plenum has to line up with whatever bonnet you are fitting as well.

cheers
Max