Thanks for all the great info everyone. If this project goes forward ( half the fun is research ) , I'd be getting a complete new as possible car so I have everything.

An auto 4 speed is a must, the 3 speed 400 does not have a deep enough 1st gear and by the time you use a deep rear axle to get any sort of pulling power, highway engine RPM is too high. At some point the truck would see towing a 20 ft box trailer so having an assortment of ratios is a must.

Late V12 used the 4L80E , which is more or less the 4 speed version of the GM 400. This gives me a choice of swapping tail shafts to match the truck. If this was using a ZF or other odd trans, I probably would not consider the swap.

As a side note, there is a turbo / supercharged later Jag that uses a MB 722.6 , this could be an interesting swap into a MB. Having at least something mechanical that bolts right up helps the project a lot.

The British transmission industry was well and truly dead by the time the XK8 came along. I believe all V8 Jags use 722.x transmissions. If you're looking for a powerful, modern engine, a supercharged AJ8 from an XKRS would be an interesting choice for 400+ HP. Be aware that the cylinder liners are not repairable, and there were timing chain tensioner issues on early engines.