Straight forward really, drop the engine and subframe together then move the new shell over the top and pull it all up with a crane, the interior e.t.c is easy, i reshelled mine in 2 days, if its your first time doing such a big job you should do it in 4.
Regards to the vin/identity, just use the new shell's i.d? if thats not an option then you will be 'ringing the new shell' which is illegal, unless its brand new and doesnt have its own identity, it isnt legal to change the identity of a shell however many people do it to other cars, on the mk1 the chassis number is on the vin plate and on the drivers turret, it's worth noting that you may be refused an mot if the turret numbers are removed as the tester wont be able to tell if the shell is right for the vin on the plate.