I have a '62 C10 with a 327, SM420, and a stock 3rd member 3.90:1 rear end.
I have no trouble running down the freeway at 65-70mph all day long, with occasional bursts of 75-80mph. Both my wife and daughter can drive and shift the SM420. Granted, it is easier for me, but I have a lot more experience with it.
My wife used to drive a Chevette with a 4 speed, so she has experience with a manual, but it's been a while. My daughter learned to drive in the '62, but prefers her Harley.
If you plan to use your truck as "a truck", then keep the SM420, it's bullet proof. If you plan to use it as a freeway driver, then I would consider upgrading.
