I would go with Nev on the impact driver. If that doesn't shift them easily, apply heat. Loctite in its various forms can be released by heating to the appropriate temperature. Then apply the impact driver again. As Nev says, the 1936/7/8 primary covers are rare. But the shape of the cush drive dome is the same as that for the Square Four from 1937 to 1959. Again, these covers are scarce these days, but you might find one damaged at the rear end.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), 54 4G Mk IV (project), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser.