Defo something not right on your Ipotesi by the sounds of it.
That final plate should fit in the basket along with all the others.
There should be 6 friction plates and 6 steel plates plus the final end friction plate. I wonder if someone has fitted extra plates at some time? When you strip it make sure you get all the plates out, sometimes you might discover a last metal plate at the bottom of the basket that is hard to see, which sould also not be there, (use a magnet to help remove if it is there).
BTW the 350 B engine uses the same clutch basket etc but only has 5 friction and 5 steel plates according to the parts books.
The springs need to be all done up or they have a habit of winding out. If the clutch does not 'lift' square you can ever so slightly grind the end of the springs to give the correct amount of pre-load.
I cant think what else could be wrong except for perhaps the wrong basket, hub or indeed plates being way too thick.
Complete strip and very carefuly check against the parts diagram to make sure it's all as MV intended is my best offering.
Good luck.
Dorian.