Actually the ZF 4 speed isn't so bad. It has lock up on third and on fourth, so economy at speed is the same as a manual and, as importantly, all the power gets through to the wheels. The HP22 and HP24 can both go in the HP22 casing (24 has a much higher torque capacity so is bombproof as far as any Rover derivative is concerned, whereas the 22 gets a bit marginal around 250 brake) so can both be easily grafted to the back of a Rover V8, and there's "just" enough room for them in the tunnel of an auto shell. The five six and seven speeders use a different family of casings and, even if you could sort out the bell housing, I don't think they are going to fit in the tunnel without surgery.
The fourth gear overdrive ratio is the same as the LT77/R380 so gives a nice relaxed cruise, and the torque of the engine really doesn't call for any more than four gears. If you must go flappy paddle this is possible on both the 22 and 24 provided you use the Jag or BMW electronic versions. There are a number of yankee aftermarket control systems, designed originally to control auto boxes behind dragsters, which allow you to set the transmission up exactly as you want it. Speed of change, harshness of change, change trigger points, torque convertor lock up points etc etc. Only problem is, you can programme in conflicting instructions which will result in a very big bang, so you really need to understand the box before you start!
Chris