Actually, if your goal is a large negative, I think that making an 8x10 print and then, as was suggested above, contact printing the paper onto 8x10 sheet film is a great idea. This way, you've got built in ND, the paper, and if the prints are consistent, the overall density range of a print will be confined to a much smaller range than a negative, and at a given enlarger height you'll always have the same exposure (save for some minute adjustments).