Mdavlee, I should have been more explicit, I was thinking about the INSIDE of the covers. It would be informative to see/measure the differences in the depth of the covers.
After doing the 450, the 555 covers don't look they would be that hard. A person would have to drill a couple of holes through the middle of the cover, then take a Dremel and open up the back the rest of the way. Essentially, you leave the sides alone and just remove the area in the middle that is restricting the flow. It looks more like you put a slot in the cover when you look at it from the outside.
With the middle of the cover cut out, you epoxy some fibreglass across the hole with some GB Weld - that becomes the new inside for the back of the cover. You then just add fibreglass and GB Weld where it needs it and shape the inside.
If the thickness of the metal in the cover is 5mm, then you have increased the depth of the cover by 5 mm when you glass over the hole/slot. However, you can go further in depth if you keep building up the outside and grinding it out. It looks like taking out the back would be more than enough, it might even be too much. Perhaps just porting the cover to the right dimensions is all that the 555 needs. If you broke through, then glass it over.
On the 555 it would be easy to find the proper dimensions - just measure the inside depth of the 562 cover. On the 450 I had to measure the area of the port opening and then work my way back down into the crankcase, gradually opening up the area of the tunnel as I went (a tedious job that is unnecessary on the 555).
Opening up the 555 covers is very do-able.