IMO 6500 RPM launches with a 3.09 or 2.66:1 23 spline and slicks in a B-body are fine. If something breaks, it's probably not because the trans isn't strong enough. They used to run them in the pro stock cars in the 60's and early 70's. Keep in mind that whether it's 18 or 23 spline, 8 3/4 or Dana 60, you do get fatigue in parts.