As you stated, size of hands and forearms are limiting factors to the vent approach.

If you don't drop the underdash panel, how do you securely reattach the vent tubing back onto the driver's side vent frame? Dropping the underdash panel and removing the force fit plastic ducting provides a clear shot to the brass knurled speedo cable connection even if you have lobster claws as hands and then putting back the ducting at both ends is literally "a snap."