Unfortnately that's not legal, at least not if you are in the UK.
No VIN number was stamped into a 1960's Mk1 bodyshell so why or how is anyone going to prove he used panels or otherwise? and why should anyone care if it is done with careful attention to detail, to restore and save such a rare car from oblivion. An awful lot of Mk1 Mini Cooper & S out there have been lovingly rebuilt or restored over the last 40 odd years around an ex-'one old lady owner Mk1 850 Auto shell' for instance, whether due to accident damage in competition or the ravages of rust............including the BMC Competition Dept. for most of the Ex-works Minis who regularly swapped shells, Chassis plates and Austin/Morris identities from one rally to another! Perhaps we should encourage the owners of such Minis to hand them all over to Gordon Brown and Peter Mandelson so they can be crushed along with all the other Classic Minis that have disappeared into their scrappage scheme and just retain the few that can somehow prove they still have their original shell?
There are differences between the 'shells.
Most notable are the brackets for the hydro pipes, location of seat belt mounts and sun visors. The front bulkhead area also changed a little with holes/captive nuts being varied between the years.