Unfortunately, we're not going to have an elegant solution for a footprint as the older Half Dome 2+ tents were 6" longer than most other tents. You have a couple of options:
Purchase the newer half dome footprint and use some cordage to help extend it and stake it out. It will be a few inches short, but footprints are designed to be overlapped a bit by the tent. This would not allow you to pitch the fly/footprint option, however, as the grommets on the footprint would likely not be in the correct position.