Thank you very much for your reply Phil, it has been really helpful.
So I ground this casting stubs. After I did, I could place the engine/gearbox as I wanted. I have to confess that my euphoria from yesterday went down a little. Actually, the MT75 fits, but it is very close to CP17 on the driver (left) side, and the alternator of the DOHC is very close to rail D2 on the right side (which is not due to the MT75, but to the DOHC configuration). About 5mm on each side only.
The picture on the left shows the gap between the bell housing and CP17 on the driver's side, and the picture on the right show the gap between the alternator and D2 on the passenger's side.
Gap01.jpg
With the engine movements/vibrations, I guess this gap is not big enough.
To make sure, I drilled the engine mount in order to have the engine mounted correctly. When the engine is sitting on its rubber mounts, the bell housing is about 10mm from CP17, but the alternator touches D2 on the right side. The third picture below shows the maximum gap I can get when I strongly push the engine to the left.
Gap02.jpg
Even if a 10mm gap between the bell housing and CP17 is enough (what do you guys think?), I can't let the alternator touching D2. I thought about 2 solutions:
1 – putting some dense vibration absorbent foam like the one used to make these thin racing motorbike saddles (EPDM foam, very good shock and vibration absorbent proprieties, and it is oil and fuel resistant). Will this be enough ?
2 – I hope 10mm between the bell housing and CP17 is enough (I can still pad the area with the same foam), but on the alternator side, I'm thinking about removing D2 and weld a new bracing made of 2 tubes like on the sketches here under. Do you think it will be as efficient as D2? Which one of these two set-up do you find the best and the more adapted to what the chassis needs at this place?
AlternativeD2.jpg
Do you have other ideas to turn around the problem I have?
All answers, comments and opinion, are welcome.
Thank you very much.