I will try to keep the answer short, and this is in general specific to the Heat. The reason our boats need a spacer, instead of a shorty, is the factory x dimension differential.
I had a Baja (H2X) and like 99.9% of them, the drive was buried at 16" XD. That was not unusual for the hull, which needs lift, and propellers available in the era.
In the early 2000s, Nordic began running a single step bottom, and raised their X dimensions radically to 18.5/19.25".
They were looking for a more efficient setup, but they didn't do a lot of on the water testing under former owner Orval Summerstedt. Several enterprising owners of these boats who DID actually test our boats with GPS speedos and slip calculators realized we had slip in the 18/20% range.
To reduce slip to a more efficient 12% range, several of us have added a 1" spacer, and began running 5 blade propellers for more submerged blade area, better acelleration,
midrange efficiency, and higher top speed.
The concept of running an Arneson drive is interesting, but you'll be "flying blind" from a database standpoint. Sometimes it is easier to go with known variables......