It depends entirely on what you want. For many the T5 conversion is a nice alternative but you have to give up the old torque tube and adapt a newer style rear end in there. It isn't all that difficult, actually. Just takes time and time to study which gear ratios you want to end with. If you're careful, you can have nearly the exact same ratios as the stock 3 speed in gears 1,2,3 and then 4th becomes your first overdrive with a 25% reduction in rpm and 5th becomes your second overdrive, useful on relatively flat terrain and with a roughly 14% reduction from your 4th gear. If you have a 261, the T5 should be a natural with it and gears 1-5 are synchronized. I agree going 65 or over in the AD truck isn't wise or safe...although there are some in the audience who will jump up and say "oh I drive mine at 70 or 75 all day long." The truck was not designed to drive at that speed. As Jerry once said "it handles like a hippo on a wet mud bank" or something close to that.
