The lifter bores don;t look too bad in terms of placement. You may not need much. I'd try them all, carefully marking where they hit or are close. Then it's time consuming, but you can mask off the rest fo the engine completely and clearance each spot. Use a shop vacuum directly adjacent to the cutting burr, and a magnet to clean up the shavings before you unmask it.
BTW - have you fit the intake yet? You may have issues using the 90° fitting on the crossover.
