Hi Tom ... sensible upgrade if you ask me!
most probable cause is the inhibitor switch - intended to inhibit o/d from operating in wrong gears. Mine does this, too, so i just remember to switch
out when changing down. BEWARE of putting it in reverse if you're having this problem, though, it's not healthy and sounds bloody awful to boot!
Unless there's any other reasons that it can operate in the wrong gears (I don't know but you can bet someone will take the bait now) the solution is
to check that the wiring is correct to the diagram, and if so consider replacing it.
Otherwise just use your car for 'razzing' like Adrian does in his Vitesse, (i hope he doesn't do that in reverse though) or fill yourself up with beer
like the good man, Don!
Btw Don ... any luck with your J-type and rotor arm 'issues' yet?