Just shoot the full 4 x 5. You can crop to 6 x 12 etc. Mask when you print via enlarger, or mask with the mat board. Make your 6 x 12 marks on your ground glass for composing.
Drawbacks....4 x 5 film per shot is maybe more expensive, but you have the option of printing the full 4 x 5 image, or any cropped variation.
A 75 mm or 90 mm that covers 4 x 5 gives a nice wide perspective on 6 x 12.