Not building a 5'er (yet), but if I did the 4X8 and 1X8, I would want to bond that long seam so that the sheet becomes a 5X8 when completed, either by an epoxy/fiberglass, taped seam, scarf joint or my newly discovered finger joint. I wouldn't want an unsupported seam running the length of the tear perpendicular to the spars. As an alternative, you could fit braces perpendicular to the spars that support that seam. Personally, I would probably look for the 5X5 or plan on additional waste from 4X8 so the seams in the roof and ceiling all run parallel. But I was dropped on my head as a child, so I usually question my own logic.
Still graced with two eyes and ten fingers (due in no small part to luck!).
Just when you think a problem is solved, an uglier result replaces it.