Axle Twist Allowance?

I've got a '66 Ranchero with a fairly mild 460, C-6, and a 28 spline, spooled 9 inch. After about a dozen passes down the eighth mile, I twisted the left axle. It was the stock 6 cylinder one so I wasn't all that surprised. I replaced them both with heavier Mustang axles and after about a dozen runs, I've noticed the left one is twisted again. I paint a narrow white stripe along the length of my axles and check them every three "post race inspections". This time it's only about a two spline twist and isn't concentrated at the end but runs the length. Is this safe to use or do I need to get out the plastic and go aftermarket? I've heard some slight twist is acceptable but I'm really not very fond of turning left into another car or the wall when launching. Any thoughts or advice is appreciated.