My Solution to adjust my itx motherboard to fit properly inside a Mac Classic case.

Motherboard is an  Gigabyte A520I AC ITX, which happens to have the good property of having the cpu close to the right (still you need to cut the metal chassis to fit the cooler).

This design also account for using the original power & reset button from the main motherboard (after keeping only 21mm of the original motherboard).

Notes

Mostly use M3 screws, except for MotherBoard that expect M4.
On left side motherboard is attached from below (M4), on right from top, this to allows to slip in the cpu fan. 

Glued the corniere to the bottom of the iron chassis (the thickest part in the air), I have used B7000 kind of glue for that. The design allows for 2mm adjustement on left/right axis, and as much as needed in depth.
 

Back cut is aligned on the 3 inner legacy port holes (external floopy, external drive and printer).
 

Attach the motherboard to the back right only at the end.

My notes on the complete project can be found here