Why make things complicated? I think that if I were you I would tell the DVLA nothing and just put it through it's MOT. If the tester notices the discrepancy between the model type on the V5 and the actual length of the vehicle, just show them the chassis number stamped on the front offside spring hanger (assuming you have not replaced it). I am sure there are plenty of hybrids around with Range Rover chassis shortened to fit 88" Land Rover bodies, which describe the vehicle as a Range Rover in the V5. I think that it's important to tell your insurance company what you have done though.
Have a look at the DVLA website. There is a points based system for deciding if a modified vehicle can keep it's I.D. If your engine, suspension system, transmission and chassis are original I think you have a very strong argument that the vehicle should keep it's I.D.