I used to proof a 120-roll's-worth of 6x9cm negs on one sheet of 8x10-inch paper regularly. Should be no problem with the Fuji 6x8 (which is basically the same).

FWIW, 6x9 (or 6x8 in your case) gives you a whole lot more negative area than 6x6 unless you plan on printing square. Cropping 6x6 gives you 6x4.5 or something similar for that aspect ratio; cropping 6x9 to the same aspect ratio is your 6x8: 27 vs 48 sq. cm. That's a major difference if you are planning on enlarging much.

Best,

Doremus