Truck towing question

I have a '67 d-100 factory 383 4 speed and it would pull anything. I added an electric brake controller and had no issues. I have since added a rebuilt 440 built to HP specifications and a 727 automatic with 3.23 Detroit Locker 8 3/4 rear and added a Scarebird disc brake conversion and it pulls anything I will give it. It is just an old farm truck but everybody asks what it is etc. A Gear Vendors overdrive would help with milage but I do not put enough miles on it to justify the expense.