It is virtually impossible to convert a Mk.3 or later into a Mk.1
Virtually the complete side of the shell from A-post to the rear is different. The B-post is completely different to allow for the longer and different shape door which allows wind down windows.
The front sub-frame mountings are completely different.
The rear side windows are a different height, the gutters are different, the back of the front inner wings are different with the door support structure changes.
The floors and central tunnel are different.
Years ago I restoring a Mk.2 Cooper 'S' by using a brand new later shell. The conclusion I came to was that to do the job correctly would involve putting a brand new shell into a body jig, cutting off the sides, removing the floors and tunnel, cutting into the front bulkhead to fit adaptor pieces, welding in new sides, adding the gutter strip, cutting off the entire A-panels and re-building as Mk.1, buying doors, buying hinges, changing/modifying front panel sub-frame mounts and sub-frame tower mounts (for twin-bolt towers). That lot would cost a small fortune.
When a full re-paint is added it would be cheaper to buy a decent Mk.1 in original condition which could be done for around £6000. I have a 1964 Mk.1 in restroration at present. The work is a lot less than the conversion and I reckon that having paid less than £2000 for a completely stripped down project, but with virtually everything for a full restoration I'll have a fully restored almond green/white Mk.1 850 for around £4500 including everything.