I thought the only two spline count choices were 27 or 32. That being said a 27 spline input for a NP208 is common and inexpensive. Plus it is pretty easy to install. No special tools needed. Only trick is to get some A/Trans "Assemble Goo". Goo helps hold the needle bearings for assembly. All you need is some RTV to seal the case back up. The 32 spline input you currently have can be sold and you will probably only be out $20. I'll see if I can dig up the flexplate number. I love my 700r4, however it took fiddling around with the governor and other things to get it shifting right.
I used an aftermarket flexplate from Summit Racing, it was an ATP Z-136.
I highly suggest the Sonnax "smart shell" sun shell. If you have input to what your builder uses I think they are superior to "the beast" sunshell. The Sonnax one is carries thrust load with a bearing and is supposed to help planetary life because of the re-direction of thrust load. Plus the shell is strong.
Other than that it is a mix of mojo and magic. I have a servo out of a 4l70... I think. It is larger than any of the v6 or v8 servos, but smaller then the corvette one. Some decent clutches, a good rebuilt converter, a good pump and a full transgo kit and I'm a happy camper.