the problem sounds like your needle, not the mains. you really shouldn't need 320's with stock pipes and no portwork, unless your gass is really, really old. her's how it works, roughly.
+1000ft = -1 size main.
remove snorkel and lid+3
remove airbox,= possibly+1
high humidity, go down
low humidity, go up
more oil in the mix= richen up
with any other variables, you could be different, as every engine is unique.
you would likely be in the range of 230-250. you would need pipes/portwork to put you in those higher ranges. if you are running pump, 90-93, then do a plug chop. always start with the mains(wot) pickup some stock needles and tune them, then move to the pilots, which shold start with 25, and the airscrews @ 1.5 turns out.
another thing that will make you think it needs to be rich is an airleak. check for those by spraying carb cleaner around your intake boos/reeds, and base gaskets while it's idling. when the engine changes, you found it. also helps to move the carbs around to find any partial crack leaks in the intakes.
the 6 clip needles usually are long and thick to lean out the low/ mid throttle, accomidating for large main jets.
also, check your silencers, as that will effect the jetting as well.