I also have a 1966 Chevy 1/2 T short bed step side, with a 350/350 HP turbo 350 auto with a shift kit, not a 4/4, and not a 4 speed stick.
If you have the stock 4:10 ratio gears, it will not be a speed demon, and 55/60 will be all the faster you will want to tow with it!
That is probably a good thing, as the stock steering is a little sloppy, to say the least!
I converted mine to a 79 Firebird front and rear suspension, left the coil springs in the rear, welded in new perch's, so now a posi rear, and the 1 to 1 power steering and disc brakes made a very big difference in how it drive's.
You will need the overload springs in the rear, for sure, and can get them off a 3/4 ton pickup from 60 to 66 and they should bolt right on, as the holes are already there in your frame! They are short leaf spring style that mount on the outside of your frame. As you load your truck, they drop down onto a perch, and work well! It lets you retain your good ride while you are not towing!
Now, for the bad, Your brakes! Not up to stopping a 5000 lb trailer, along with your heavy pick-up! You will have to have good brakes on the trailer for sure, and, if it was me, i would be seriously be thinking about upgrading the braking system on your "ol-66" The single master cylinder and drum brakes, will get hot very quickly, as you probably already know!
It will pull your trailer very well, but stopping it, another story!
My son has my old 66 now, and is in the process of installing a new motor, he lives in Central Illinois, and if you have any questions, i can hook you up with his #!
Good luck, and i will be following your progress! Rail!