Fabtech makes a kit $$$ Its a pre runner set up you are looking for.

As far a converting your 2wd to 4wd you are in for a long hard road. Best bet would to swap you body on to the existing 73 4x4 parts doner, if you have the frame w/running gear.

Here is some 4x4 conversion info copy and past for you.

There is nothing nuts and bolts about a 4wd conversion, there is not a thing that is bolt on or swappable. The engine cross members are entirely different, and this means that if an un molested truck is what you are after, it will be impossible to create one with a 2wd platform. The 2wd will have to be lifted quite a way to clear the front diff.

2wd trucks are coil sprung and most F250 4wd's are leaf sprung. None of the suspension items are even close to the same, so you will be fabricating or modifying some original 4wd stuff to work. Steering systems are entirely different as well, so you will be upgrading that too.

My last conversion with leafs ran me several thousand dollars, and that was with the work that I performed for free. The difference in sale values between a 4wd and 2wd are minimal, and the 4wd conversion is hardly justifiable. How far away are you looking? I pick up F250 4x4's for reasonable money. Consider the cost of shipping, and then consider buying a west coast truck with no rust, and I can guarantee that you will be time and money ahead.

Well the frames are similar, with a few exceptions. The frame horns are slightly different, and the frame widths aft of the cab ar different from 73 to 77 trucks. The frame rise is also slightly different, but this is not a show stopper and does not make much difference one the fabrication begins.

Not that the important question is whether or not you will be using a coils springs or a leaf spring. It is possible to install a 78/9 radius arm and suspension. Not a big deal really, and with a standard cab this is reasonable. Not quite as easy with a super cab and or crew cab.
The 2wd frames are not cut for the shackle pivot, and they do not have the provisions for a spring hanger, but again, this is not a deal breaker. Installing a pivot is just a tedious process. Accuracy is key, but I am not telling you anything that you do not already know.
Ok, so next is the transfer case. You have two choices, Install a married case, and this requires the proper transmission. Might find some difficulty making or modifying an output shaft to accommodate the early C6 and a married transfer case. So then the next option is to find a divorced transfer case, and install this. You will have to make up a cross member, but then you will have to get creative with the shifter linkage. See the 4wd trans will have an adapter or tailshaft with a pivot for a typical 205 shifter.
Fabrication is the only limiting factor here.
Next and final factor will be the speedo. See your speedo works from the trans, so that being the case, you will have to get a speed reference form the t-case. If you do not, the speedo will read twice as fast when you put the case in low range.

As mentioned the steering box will have to be relocated, and the truck will require some lift. That engine crossmember has limited clearance so you will be looking at some lift to even clear it. The other option would be to remove it and replace it with a tubular unit.

This is a heck of a project. Not unreasonable, but a project none the less.
2500 bucks for a 2wd might get expensive. I paid 1300 bucks for a 4wd 75 last year, and it was already 4wd with 38" tires, and a strong 460, A/c, and plenty of goodies. I have made these conversions, and can tell you that it is a chore.