Converting Naval Weapons for use on land?

[Rainbow Six]

Guys, would appreciate your input here.

I've been doing some work on the south west of England lately, specifically Plymouth, which is home to a major Royal Navy base and am looking at the possibility of HMG forces stripping weapons systems from immobile ships for use a) as defensive platforms and b) as offensive weapons to use against the Duke of Cornwall's forces.

Does anyone have any thoughts on how practical it would be to take a surface warship's major weapons systems (e.g. 30mm oerlikon cannons, 20mm Phalanx CIWS, 4.5 inch gun) and convert them for use by land forces? Could these be used as a platform for gun trucks, or would they be too large? And would the engineering resources required be beyond that available after the nuclear exchange?

Cheers