[David C]

I am indeed in the UK.

I have gone for an A400 42RH transmission. It's a fairly straightforward bolt on replacement. Needs different flex plate and lower cover for torque converter. Also a small ear had to be removed from the block to get the starter motor to fit.
46RH are a bit easier to find and are a bit stronger. Most of the work is in modifying the gearbox crossmember and transmission tunnel. Article to follow...

The Poly engine in the 409 and 410 is internally balanced so works with the lock up torque converter without any modification. I think I previously saw that the 383 is also internally balanced but that would need to be confirmed. If so it should be fairly straightforward to bolt to a 46RH with a suitable adapter kit.

As long as the engineering is done well I don't see a problem with a GM unit. Wilcap make a suitable adapter. I think you will need an electronic adapter if the GM 'box is electronic.

It would be good to hear from others on the forum whether they would welcome an extra gear and better fuel economy or would be put off by non original gearbox. Also, once it's non original does it make any difference whether it's Mopar - developed from the original - or from another manufacturer.