i recently got 1973 f100 2wd truck. i want to make it a 4wd and i was told you have to change frames. if i was going to change frames would any 73 to 79 f series truck frame be able to have the 73 body bolt right on to it? if not what truck would the frame modifications be easiest to do?

 

Sell your 2WD and simply buy a 4WD. You'll come out miles ahead starting with what you want instead of cutting and fabbing what ya got.

IMO, the "best" configuration for a half-ton is a '76 since it is the only year 4x4 with disc brakes, integral power steering, and an FE engine.... I'm not a fan of the 351M/400.

 

The easy route is do what silver said, but if you have to have a 4by with that body, then yes get a 78/79 4x4 frame and just swap bodies.

78/79 gives you front disk brakes, rear gas tank with gas filler flap.

Doing a 4x4 conversion is not for the light hearted, a body swap is way easier.

 

If you do a frame swap, be sure to get the title that goes with the frame.
In most states the frame VIN is the one required for title purposes.

 

Or you could use my favorite saying:

"There ain't much a cuttin' torch and a welder won't fix." Of course that wouldn't be legal, but if you did a good job, how many people could tell?

 

Here is some conversion info for you to think about.

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.
Well the frames are similar, with a few exceptions. The frame horns are slightly different, and the frame widths aft of the cab are 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.
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.