My brain is now on transmission information overload !!!
But I think we're there, now.
We started off considering using a simple BOP TH400 . . . . then deviated on to a Switch-pitch '400 . . . then, after multiple discussions and hours of reading . . . looked at installing a 4L80E !!! - Modern or what !!??
Simple physics prevailed in the end . . . after doing the calculations the 4L80E's would deliver an engine RPM of only around 1680rpm @ 70mph . . . meaning the engine would never be at it's ideal power and efficiency band at motorway speeds.
By comparison the TH400 (SP) would be around 2250rpm at the same speed with its top gear ratio of 1:1 (the 4L80E has an overdrive of 0.75:1).