Masking a Pentax 67 frame to make it pano creates an image just a wee bit bigger than what an Xpan creates. Or Noblex 135. Or Widelux. Or Horizon Perfekt U500. Just a few mm wider.
The Xpan is so small (in comparison), so much fun to shoot , with double the number of shots per roll, that unless you have no choice but to use the Pentax 67 there are far better options.
Why not just shoot it as a 6x7 to begin with? 120 film is cheap enough, and it is tensioned in the Pentax very flat, consistently in focus; and you can
crop it any way you wish afterwards. Besides, you have access to quite a range of focal lengths as far as lenses are concerned. But if you are a panoramic specialist, do whatever is appropriate. If so, I'd want something substantially bigger, like 6x12 or 6x17.
I take the Pentax 6x7 when I want to go light as apposed to the RZ67. Went to the arboretum with the family (wife and 2 year old) and took the P6x7 with me. No problems.