Best, easiest and cheapest are usually 3 different things.
Best (fairly easy and not cheap) would be a 4R70W with an aftermarket transfer case such as a Stak or Atlas.
Easiest is to use a 2WD transmission and leave off a transfer case. 2WD hack job. But it would be nearly pointless.
I will simplify it to 2 transmissions. AOD and 4R70W. The quick and cheap way to mount either is to use an NP205 transfer case behind a factory 4WD transmission. AOD is cheaper way and the 4R70W is a better transmission. Both can be done without opening the transmission. Keep in mind you are liiking at a 15 to 20 year old AOD which had poor engineering in the early ones. The 4R70W is in the 5 to 10 year old range.
Any way you cut it requires driveshaft modification.
My recomendation for a low buck, quality transmission swap. 4R70W from AWD Explorer V8 ('96 to '01 5.0 V8 only), Late 70's NP205 (can't hurt them as long as there is oil in them).
All overdrive swaps will require a miniumu of extensive modification of the transmission crossmember or fabrication of a new one.
Be warned this will quickly become a repeat debate thread over transmissions. There are also other factors like how good of a tranmsission do you want? You can pick up a $50 junkyard bomb from an '85 F150, drop it in and hope it lasts a month behind a 5.0 mustang engine all the while it short shifts as the governer springs are for a truck engine and the torque compensation is for a 2 barrel carbed gutless wonder.
There are countless threads on this. Many writeups in the tech section as well.
And while we are at it, what size tire and what axle gears are you planning on running? An overdrive may be pointless for you anyway unless you regear your axles. Most of the mid to late 70's seam to have 3.50 gears in them. That gear and an overdrive don't work together too well unless you are looking at running stock tires sizes. Even with 4.10 gears an overdrive isn't going to be any good over about a 31" tire.
Expecting massive fuel economy gains? don't.