I figured that you were doing that, finding out what the M90 will do.
I'm still waiting on a couple of parts for my 4R, but I'm also considering trying another VB, from a later car instead of a truck. The popular VB plate holes drilled evidently don't do well in a truck VB. So I can take a chance with mine, tame down the hole sizes, or start with a car VB to begin with.
I have a good feel now for what parts are best to work with, it's actually not much beyond the stock 99+ parts. The 4R70W is very strong, internally, compared to any later trans but the big truck stuff. These are not in the same league as a towing trans, what you get in a turbo diesel or duallie etc. Unfortunately Ford made the Explore V6 trans from a 4 cylinder Thunderbird trans, and the later five speeds have similar strength internal parts.
You should gauge a trans by two things, the VB controls(imperfections or flawless), and the internals. If the internals are strong enough for the loads, then you can usually make the VB etc, work well enough to survive. I think the 5R trannies are all lacking greatly in internal strength. The actual size of the clutches and drums etc, they don't have the capacity to take much power past a certain point. That point is debatable, and I'm not the expert to say what that is. I'd accept the word of the guys who are building them for high performance.
So when you have yours built, don't pay for any odd stuff that is extremely high. I don't know what's out there for the 5R, but most of it will be for getting the VB and controls to work right. Concentrate on that, and keep an eye/ear out for any upgrades that may come out before you find a limit for your trans.