Think the M5 is going to be the problem. Any M5 fixing that long is rare so the chances of finding a coach bolt are not good.
If there really is no other option then you could get some bits of studding welded to shorter bolts to extend them (assuming you don't need full thread and there aren't too many). Otherwise you may need to go up to M6 or M8 or change to a hex or other std head to get M5 x 80