It's a bad idea to cut strips down any smaller than they need to be to fit the LS-8000/LS-9000 carrier.
Measure the average distance from frame start to frame start in mm. Use that in the Input menu as the "frame spacing" value (Q: why would this vary much on a Holga? Aren't you using red windows to set the frame?)
In the Crop menu, set the crop size to 6x6 and set auto-offset.
Don't bother locking the exposure on the Input menu - because auto-offset is going to do a preview on each frame anyway. You might as well get the benefit of frame-by-frame calibration.
Set this way, unless you have frame overlap, Vuescan will scan an area bigger than the expected frame, set the frame automatically, scan and move on. This will work even for old cameras with uneven spacing.
Make sure you use a "buffer" value of 5-10% to eliminate exposure slipups from the inevitable white or black border that Vuescan will pick up at "6x6."