Listen to Hotrod... Anyone who drove aggressively or had a teenager in the 60s knew about the Powerglide "slip". In other words, if you drove aggressively, it was only a matter of time once you started feeling that slip in your glide. My sister personally proved this in 3 months from her 16th birthday on a 67 Chevelle. When the THM 350 came out in 69 they quickly outsold the glides and for good reason. If you wanted power and performance in 67 you drove a stick.
In recent years, I've had a 57 Bel Air, a 61 Impala, and now, a 68 Camaro with glide. They've all done fine, but my purpose in each of those cars was for cruising and shows. If I planned to do more on the performance side, I'd at least put in a THM350.