The front suspension brackets are tricky. All the conversion companies pretty much make their own brackets. The conversion I'm doing uses coil springs and a late model coil spring front axle/swing arms. Done incorrectly, death wobble will cause you grief, so I'm going with someone who already has it figured out. Agile does TTB conversions, but they are pretty expensive and use the Dana 50 TTB axle. Timberline does a 4-link coil set-up that works pretty good. Quigley and Sportsmobile won't do old vans anymore. U Joint off road does a leaf spring kit, but the articulation is reduced (compared to coil springs). There are some IFS options for Chevy, Nissan, Transit, and Sprinter vans, but I'm concentrating on the E350 that I have.
You have to deal with the lug pattern difference, 8 on 6.5 stock vs 8 on 170 for the new axle. You can either change out the rear axle or use adapter/spacers. The pick-up axle spring pads are different from the van, so that needs to addressed if you change the back axle.
The fuel tank needs bobbed (about 3 to 5 gallons worth) to make room for the transfer case. While you're at it, the transmission needs to be reworked with a 4x4 tail shaft. There is another way to do this, with an extended adapter, but that really cuts the fuel capacity.
You're committed to a 4" to 6" lift, to give the front diff room to move.
Shops are required to maintain all factory safety equipment (RSC, TPMS, 3 or 4 channel ABS, etc.) that the van comes in with. This can get tricky switching axles between years. U Joint has a problem with the RSC. If you use the (correct) dropped pitman arm, you get a warning light. The stock arm doesn't give you a light, but the steering is reduced.
I'm NOT knocking any of the conversion shops, I just have my own preferences about which one I want to use.
Hope this answers your question!