The Boge struts (earlier cars I think) were designed to be re-buildable. The bottom is flat, with a disc and nut holding the damper into the strut via a stud on the bottom of the damper itself. As far as I know, Boge inserts have been unavailable for many years.
The Spica struts (later cars?) were not designed to be re-buildable. Instead, the idea was to buy a whole strut! The damper is crimped into the strut just below where the bottom spring pan is located. Spica struts are unavailable. The bottom of a Spica strut should be rounded, with no bolt or nut.
At various times different companies have made inserts which could be fitted to both struts. I haven't had a Sud for years now, so don't know the current supply situation.
Best bet is Koni inserts for still being in production (though probably special order and expensive).
If you have Boge struts, and you can source an insert, should be fairly simple.
If you have Spica struts, which have been used in a racing application (as appears the case), they have probably already been modified. This requires grinding away the crimped metal to release the old insert, and then drilling a clearance hole for a bolt to hold the replacement damper inside the strut.
The last Sud I ran had modified Spica struts, with Koni sport top adjustable dampers, and Eibach springs (fitted with standard Spica spring pans).
I think your best bet would be to try to source a pair of struts, and then source suitable inserts for them.
Think your top mounts have been modified as well.