So I have a 6L XJ12 and a 6L XJS, both using the 4l80E trans which I think is fantastic. But I've also heard that these engines really wake up when mated to a manual. I'm wanting to have fun with mine, neither are show cars so I can.

I read somewhere that the old TH400 had a special housing when mated to the 5.3 but when Jaguar switched to the 6 liter V12 / 4L80E, they kept the GM bolt pattern. I don't want to drop $5000 on a kit but I might be interested in dropping a few hundred if I can get away with using an NV3500 5 speed. This was used in GM Trucks in the 90's.

My understanding is that it's basically a Getrag G290 which was used in the XJ40 and X300. Has anyone looked into this? I'd love some feedback.

 

on all the 4.0 models, and 3.2 from 1993 onwards.

also, you will need a lot of other parts besides simply the gearbox.
manual prop shaft
manual pedal box with clutch master cylinder.
clutch & flywheel.
clutch hose.
transmission mount.
manual ski slope.
Link loom for the reverse switch.

you may well need to do some bypassing to the ECU to make it think it’s still talking to the TCM.