I converted my 4R100 from 2wd to 4wd at home for my DIY van build. I'll say that doing it to save money is not the right reason. I did it because I really wanted to rebuild an automatic, and I budgeted money in my build to repair or replace the transmission in the event that I botched it. So far she runs like a top.
Some advice if you choose to go this route:
Buy the ATSG manual... it walks you through the whole process. Read it cover to cover, more than once, before you pull everything apart. There are also some long tear-down and rebuild videos on youtube that I found to be helpful which I watched well before I pulled everything apart.
I used a Transtar Industries deluxe rebuild kit with borgwarner frictions and steels, metal clad seals, and everything else I needed and it worked great. You'll also need some special assembly grease that's tacky to help hold components together, but is designed to dissolve in ATF. I used "Dr Tranny's Assemblee Goo".
You'll need to source the 4x4 output shaft and tail housing. I bought a 4x4 output shaft with the parking pawl gear pressed on, but ended up having to swap it with my existing parking pawl gear to get the output shaft snap ring installed. It was only 5-7mil thinner. That's an example of the kind of stuff you may have to deal with that you don't expect beforehand. The 4x4 trans oil pan is a little deeper and the pickup for the transmission filter is also longer to go with this pan. I suggest looking for a pan with a drain plug as not all of them include this and it will make fluid changes easier in the future.
You'll need some specialty tools. You'll need a clutch compressor tool and another tool to hold the clutch drum shell assembly together while you lower it into the case at minimum. I bought a cheap compressor tool and made my own shell assembly tool. I used a harbor-freight transmission jack to remove the transmission. I installed better bearings on the wrist mechanism of the jack to better support the load. As I don't have a vehicle lift, I converted the front axle and rear suspension lift first to raise the vehicle, then removed the transmission and I still had to jack up the van to get the transmission to clear underneath the frame-rail. An engine hoist was very useful for moving it around.
Cleanliness and organization are absolutely critical. Clear a bunch of table space and put down butcher paper, or something similar. Keep everything in order as you remove it. Guys that do these rebuilds day in and out can tell the difference between all of the thrust bearings and seals, but I would have had a hard time if I let them all get jumbled up. I also used plastic bags to cover everything at night so dust and debris did not settle on my clean parts. You will get transmission fluid everywhere. Be prepared for that.
As for upgrades, your transmission is out of a 7.3L so I would expect that you already have 6-gear planetary assemblies and other beefier components. I want to say that the 4R100s on 5.4L vans ran 4 gear planetary assemblies for at least some of them. The 6.8l (which is what I have) is somewhere in the middle from a strength-of-components standpoint. From my reading, I get the impression ford made a lot of little changes and tweaks over the years for these transmissions and they are a little different depending on your engine. I replaced the snap ring for the intermediate overdrive cylinder with a new spiral snap ring as these have a history of popping out on some transmissions. I upgraded my torque converter with one that had a billet cover multi-clutch TC lockup and brazed impeller/turbine vanes. I also installed the largest Tru-Cool Max transmission cooler. I flushed out the in-radiator transmission cooler and ran an extra in-line filter for a few thousand miles to catch anything that may have either gotten in during the rebuild or anything I couldn't flush out of the existing cooler.
There are shift kits that involve drilling out holes in some valvebody components, and some of the specialty transmission builders can swap different components from different years such that they can add extra clutches into some of the clutch packs. I decided that was too much for me to tackle for my rebuild. I don't plan on building the heaviest rig and do not plan to tow, so I expect these things may not be that important for me. Instead I focused on using good parts and spending my money on things that will help keep the transmission cool.
I found the whole experience very fun. Its been successful so far and that's probably due to me being thorough and methodical... but I'm not ashamed to admit that there's probably a healthy dose of luck in there too. If you want to get a tiny dose of what this would be like, try rebuilding your own 271 transfer case. If you don't like that, then I would recommend you leave the transmission for a pro.
