There is a bit of confusion going on here. Some not so good information presented as well.
Ford bellhousings varied a lot depending on what engine they are bolted to. But the bellhousing to transmission is very consistent. So are most of the input shafts.
What this means is you keep your 170 bellhousing and clutch. You don't want a bellhousing off one of the big six engines, that was wrong advise.
The NP435 you want is a Ford transmission. NP435s were made for GM, Dodge, IH and others. But the input shaft and bolt pattern to the bellhousing are different. So no way to bolt them to your engine or work them with your clutch. The comment about pulling one out of an IH school bus works because it was removed from behind a Ford engine.
There are a few other details mentioned above that are also correct. Get a 2WD with the 28 spline output and square bolt pattern. The 28 spline is needed because that is what the input to the transfer case uses. The 4WD version is 31 spline. Some of the 2WD are also 31 spline. I think the change came in '79? I really can't pin down which ones will be 28 and which will be 31. Take the nut off the output yoke and count. When all that is right, it should be the square 4-bolt tailhousing bolt pattern. the tailhousing comes off and adaptors are used to mate the Bronco D20 transfer case.
For putting the D20 transfer case, there are 2 simple ways that work for you. One is a whole new adaptor housing. Advance Adaptors has been making them for decades. There is another way, you use the tailhousing from your I6 transmission and an adaptor plate that spaces the tailhousing the right distance and gives the correct bolt pattern on both sides.
There are a couple other details that are confused as well. You WILL have to relocate the transfer case back about 3". That is because all the NP435 conversions are based on the V8. The V8 is shorter and wider. The I6 engine being longer has the clutch about 3" more rearward then the V8. The shifter for the transmission will be 3" toward the rear. Don't use other's dimensions for cutting a hole in the floor or off the shelf tunnel covers, they will be wrong for the I6. The transfer case shifter will follow and also need a new hole about 3" back. The rear driveshaft will need shortening and the front lengthening.
You need to be careful about the advise when dealing with the I6. Most people are dealing with V8 engines. So the I6 specific details often get lost or are not known. Even though you stated I6 in the beginning that gets lost after a reply or two.