If you don't have some special tools for such spiral drilling, I would suggest getting a practice log, to do whatever test cuts may need doing, before committing to the usable/project log.

Not having special tools, I would attempt the following on a test log, first: Cut the log such that, looking on end, the cut lines look like a tic-tac-toe board. Cut (hand cut?) the helix around the center section, with some partial cutting on the corresponding mating sections, then glue the log back together.

A saw mill could best slice the log, but most saw mills need the log to be at least 4' long, so the mill can grasp the log securely for the cutting. Slicing a 7' log will give you a practice log and a project log.

Sonny