Can anyone advise roughly the time it should take a competent workshop to remove the old GM400 box and replace with another?. Just the work for the swapover. Nothing else.
Car is a 1977 V12 XJC.

I have done this on my XJS. The exhaust has to come off and the propshaft, then engine supported by a beam across the wing gutters, chained down to the rear engine lifting eyes, say two hours max. Then, car on ramp, gearbox supported on a transmission jack, gearbox to be unbolted, cooler lines undone (NOT easy as dispstick on the way), another hour and a half max. NOTE the top fixing of the two bolts securing the starter motor (which also are gearbox to engine fixings) is a special 12 point-headed 7/16ths bolt which MUST be, and can only be, undone with a double hex (12 point) socket. If anything else is tried to be used used the bolthead will be ruined and you will be in a world of pain! Replace with new box, another hour max. Replace exhaust proshaft etc, another hour. Add one for unforseen circs and total should be: 6 or 7 hours or thereabouts, MAX. If they have done one before, far quicker.
A specialist autobox place will do it even quicker.
This thread may help you see what has to be done:
https://www.jaguarforums.com/forum/x...ox-xjs-221677/