I've been doing a bit more research.
One option - I thought - was to use a ZF 4HP 22 from a Range Rover etc. It will of course fit the engine. In that installation there is a transfer box bolted onto the back, but this can be easily replaced by the tail housing from a RWD car using the same box - Jag, BMW, etc. The output shaft is the same.
But there's a big snag. Rover engineered the Range Rover so both engine and transfer box are in the same place regardless of auto or manual. And a 'basic' 4HP 22 is rather shorter than their manual. So their answer is to make a longer bellhousing and space the torque convertor off the drive plate by about 2". Which means the long bell housing would foul the bulkhead in my car by quite an amount.
Rover made a RWD V-8/4 HP22 combination - for ambulances and Sherpa police riot vans. But those gearboxes are pretty rare and I don't know if they used a 'normal' bellhousing/torque converter mounting or the RR version. If anyone knows I'd be grateful to hear from them.
There's a 4 speed 'version' of the GM 180 as fitted at the moment to my car. The 4L30. Used by Vauxhall, etc. But it seems that the bellhousing on those has extra oil passages in it so would be difficult to swop. They're also an ECU controlled box - although aftermarket ECUs are available to use with dragsters, etc. But of course would add enormously to the cost.