Welcome to Stovebolt Ed!
The 4:11 ratio was the smallest that came on your vintage truck. There are a few options. Keeping the enclosed driveline will prevent having to change the transmission. I do not think the 3:55 gearset will fit in your rear axle. That being said, I have installed a third member out of an AD truck in your vintage. If you can do that, then you can get the 3:55 gears and be on your way.
Your truck will do highway speeds now once everything is in good working order. However the noise may be intimidating!!!
