Tripple webbers (properly tuned) can only ad 8-10 horpose power above 6,000 rpm. that single downdraft is not recommended at all. You lose power from the stock SU carbs. Side draft SU carbs with the right needles from Ztherapy are the best intake for an L6. Especially if you only have the L24. Tripples won't do a thing for an L24. You need the larger L28 to take advantage of them.
How could you need 6 intakes when you only have probably one 2.5" exhaust pipe. You have more air going out than going in anyway... Go with dual SUs or fuel injection