I have fitted them to several Mk3 doors over the years.......you have to mount the lock upside down in the door so the lock mortice goes into a small slot cut in the pillar, where it curves out very close to the door frame. The window guide channel inside the door also needs a slot drilled/cut to clear the lock body but without interfering with the winding window glass when it drops down. Difficult to explain without photos but it can be done with careful measurement.....or practice on an old rusted door.
Or you could also maybe swap it so the deadlock body sits in the rear 1/4 panel side?
Originally designed by Yale, the ‘M69’ is well-known within the security industry, where it is used to provide additional protection for motor vehicle doors. There are five lengths of lock cylinder to suit different door thicknesses, fitting into LH and RH standard cases. This lock was once fitted as original equipment to Rolls Royces.
Operating bolt throws into striker in door post, and is held positively by action of lock cylinder.
Key is turned through 360° to throw and withdraw deadbolt. Keys can be withdrawn in either mode. Bolt withdraws fully flush into case.
With components separated, case is sunk into door-edge, mortice fashion. Cylinder is inserted through aperture cut in door panel and is engaged into top face of case. It is then screwed fully into position using two holes in lock face. Finally, set-screw on flange is tightened, and pegs may be driven into face to seal fixing holes. Striker is fitted to door-post, and a trimescutcheon is provided.
Brass pin-tumbler lock with up to 1000 combinations. Unshuttered. Brass keys, nickel plated in ‘SX’ series
Chromium-plated throughout.
Lock and cylinder are brass, case is zinc diecast with steel cover plate and brass extruded bolt. Steel striker and escutcheon.
Standard key cylinder
Vehicle Security Locks:-
https://advancedvehiclealarms.co.uk/physical-security/