Robert, I have been using the El-Nikkor f5.6 135mm for my 4X5 negatives for several years. It covers the 4X5 format and enables 16X20 enlargements on the baseboard of my Omega D-2 enlarger. I have never tried the combination, but I think I could make an 11X14 enlargement on the baseboard with a 6X9 negative and the 135 lens. The D-2 has a relatively short column, compared with the newer D-5XL, so the 135 lens is a compromise focal length with my set-up. If I were using the longer column D-5 XL, I would use a 150mm lens for 4X5 negatives. I do not believe that you will be able to get 5X enlargements on the baseboard with a 150mm lens and a 6X9 negative, unless you really have a long column on your enlarger, or drop the baseboard. You could project onto a wall, but that's a hassle. It would be more practical to use the 105mm lens, that is ideal for the 6X9 negative. Also, the higher you raise the enlarger head, the longer the printing exposure times, and the enlarger then becomes prone to vibration.