the only way is to make new mounts in the desired position and strengthen the floor around them and have all this mount off the original rails so to keep its strength.
You are best to consult an engineer, preferably the one who will finally pass it. Remember moving the springs in is going the have the same effect on handling as it would if you raised the car. All of this the engineer will have to find satisfactory before it will be passes

My question to you is, can you fit the desired rim and tyre inside a bigger tub without having it scrub on the rail during diff movement, all while keeping the tyre within the wheel arch or flare?
If not, just moving the springs in isnt going to help
On my 1000 wagon I drilled out the rails and moved them in. If you dont go to far and keep it looking original, no one will ask any questions at rego time unless you tell them.