88 E350 to start with-
It has a good working C6, but really needs another gear. I am kinda on the fence regarding how to get this extra gear though. It also needs a turbo.
An overdrive unit will cost about $1500 at the cheapest by the time it's done, and will leave me with an unlocked torque converter and known failure-prone flexplate that is NLA.
An E4OD will cost twice that by the time I'm done, but will have the locking converter, stronger flexplate if I go the six stud route, and more easily changed shift characteristics than the C6. It will be about the same strength once built too.
A ZF is doable, and gets rid of a lot of failure points, but it's a van and I wear size 13wide boots. I have toyed with an air actuated clutch, and it's doable but slow so may get interesting in traffic. This will be the cheapest route, probably under a grand, but not easy and will take some learning to drive.

The other issue is the turbo. If I stay auto, I can modify the doghouse to carry the rearward point up as high as the existing higher level to make a bunch of room, insulate it very thoroughly and even gain cupholders, junk tray, etc. Put the turbo over the bellhousing and everything will have plenty of room.

If I go manual, I will need to find an extremely rare turbo setup, find another spot to put the turbo, or stay n/a (not a great option). The shifter will be through that lower section of the doghouse, so I can't move anything towards the rear. I'd either have to move the batteries and put the turbos/turbos up front, or mount them underneath somewhere with a lot of extra plumbing.

My personal preference is leaning towards the E4OD, but I'm not sure about what flexplate to use, or if any other parts will be needed, or if it'll need a different starter, etc.
Does anyone have input for this, or anything I missed here?

 

I've bolted my e4od to a c6 flex pate and the other way around, because that was what was balanced with the engine, but I don't know if it matters. Even with a turbo, I doubt you will make enough power to break a flex plate, my 4-stud has been holding up to an r&d 110cc pump for a month now.
As for starter I have 4 that I've rotated trough my 5-speed,c6 and e4od as needed, there is nothing special about them, they all bolt to the adaptor plate on the motor not to the bell housing

 

I thought the four stud flex plate had an issue with the center cracking out, more from age and vibration than from actual power, and it is NLA. There is a spacer that can be used but it's very rare.
I thought I had read that the six stud off a PSD could be used, but looking around the PSD seems to use a slightly different bolt pattern on the back of the crank.
I just want this thing to be reliable and not have to worry about it.

 

Im dropping my zf for an E40D for my .02