1968 used 31 splines and 69-70 used 28 spline axles in the Type O. I suggest getting a set of Mosers, strong & NHRA approved, about $300 from
www.quickperformance.com (515-232-0126). The price isn't bad compared to the cost of a weaker stock axle, and you will be protecting your quarter panels. You can upgrade to 1/2" diameter studs (actually bolts that thread into the axles) and still use your current wheels, just new lug nuts.