OK Guys,
If you get a set of gears to bring the ratios closer and give also a higher first gear from a supplier in the uk what you get is a new main shaft,the one the clutch fits on, new gears for this shaft and you use standard 350 gears to make up the box,fairly simple and no engineering. The 550 and also some of the late 500s have a thinner 5th gear,the one at the sprocket end of the shaft because the 550 hase a direct oil feed to this bearing,also on a 550 you would need to use the clutch actuation on the sprocket end of the shaft. There are hundreds of these boxes in use over here,much cheaper than a full nova box.I Have personally used this setup in my 500 for 3 years and used the same on my two 350s in the past.Hope this is of some help.I must say for racing it's the best tuning going,increases lap times a lot.
You will need to use the 350 clutch centre as the splines are different.
I may have some tuning secrets but only by private email.
Peter c