MK5 games physically cannot run on MK6 mainboards (likewise MK6 vs. MK7), they are all completely different hardware (Acorn ARM250TG CPU vs. Hitachi SH-4 CPU vs. x86-64 CPU), maybe you meant cabinets rather than mainboards.
Other components inside the machines are identical between MK5 and MK6 however, such as the NSW LAB comms board. Not sure about US vs. non-US driver boards though. In Australia, I have also never seen an Aristocrat machine with a bill acceptor that isn't a GPT e.g. never a JCM which is used in virtually every other machine.
The LAB board is only for NSW software (and maybe MK5 Peru/Venezuela for the games that don't use the USA platform), it isn't used for any other region. In fact, some MK5 games (NZ?) will fail to boot if it's installed.
MK5 boards are not compatible between US and non-US versions as they have slightly different components, which is apparently just enough to stop the software from being compatible with the wrong board. Additionally, NSW MK5 boards only have one battery, with the other two battery slots not even having the battery holder fitted from the factory. The original MK5 board aka S2 (not to be confused with "Series 2" cabinets aka MVP) is from 1994 whereas MVP boards are from 1998. There are minimal changes between MK5 and MVP boards and both are compatible with all games (except US vs. non-US).
MK6 mainboards are identical worldwide and can be interchanged. As with MK5, there are two board revisions, the original MK6 (2000) and the MK6-XP (2002). Basically the only difference between the two are the EPROM sockets, with the non-XP boards originally using four 1MB system chips and having only four 4MB game EPROM sockets, thus almost always needing a System EPROM board, if not also a Storage Expansion board, so that they can actually run modern software (newer system software uses two 2MB chips). A quick way to tell the difference between boards is to look for a small gap between the EPROM sockets - an early MK6 board has 4+4 chips whereas an XP board has 2+6 chips.
Some non-US MK6 games can only be used with a 16-button cabinet (for all intents and purposes this also includes games which have 15 buttons e.g. a large bash button in place of the two rightmost buttons). Technically they are still playable in a 14-button cabinet but the the button wiring will need to be reconfigured so that the more important buttons can be used, for example removing the lesser-used buttons like the third bet and/or line button if the game doesn't need them for other purposes. Xceed cabinets are based on MVP but have the extra two buttons above the monitor, making them compatible with Xcite software.
MAV500 art glass is not compatible with Aristocrat 540 aka MKV/MK5/MVP. The 500 and 540 refer to the widths of the machines (in mm), with MAV500 art glass and button decks being narrower. MAV500 art glass is also significantly thicker than MVP540. MK5 and MK6 artwork is almost always slightly different in design even for the same games, or at the very least the MK6 art will have a newer copyright. MK5 art tends to have transparent parts to make it look shiny, whereas MK6 art often looks flatter or pastel and has a tendency to fade over time, as with MK7 (I'm yet to actually see faded MK2.5/MK4/MK5 art). Xcite belly glass is the same size as 540/MK5/MVP/Xceed but is thinner, allowing it to bend to fit the Xcite's curved belly panel, although the part numbers for the belly art are identical for both types. Regular MVP art glass will bend to fit an Xcite but is a tight fit and may cause damage. Xcite top box art is a different shape thus is not compatible with MVP/Xceed, likewise the top trim (the small artwork above the monitor).
MK5 and MK6 button labels are completely different for both US and non-US platforms. US games have red line buttons whereas non-US are white/clear for both the bet and line buttons. Non-US MK5 buttons are transparent whereas MK6 are white, while US MK5 buttons lack the suit gamble and 243 Way/Reel Power buttons as these features were only introduced with the MK6 platform, despite being on non-US MK5 machines since the mid-1990s (case in point, the very first Reel Power game was released in 1996 and called none other than Reel Power, which is a 243 way game with the suit gamble instead of only red/black). Additionally, the red/black double up and feature buttons on US machines are located on entirely different buttons compared to non-US.
Xcite buttons are Reuleaux triangles (sometimes called trilobe buttons, especially when referring to the bash button on later machines) rather than square thus the labels will not fit MVP buttons, however they will fit non-wide MK7 (Gen7) Viridians with the exception of the MK7 bash button which is smaller than the MK6 Xcite bash button. FWIW, Konami Endeavour/K2V and IGT I-Game buttons are the same size and will fit a MK5/MVP, although the labels look completely different and some buttons (notably with US MAV500 machines) have rounded edges instead of square. Early MK5 (MKV, "series 1" etc.) button shrouds are slightly taller than MVP (MK5 series 2, MVP, Xceed etc.) and will look out of place if both button types are used together.
Another observation: MK2.5/MK4 (MKIV) machines using the early MKV cabinets have a different spacing for the mech meters, with MK2.5/MK4 having 6 wide-spaced cutouts instead of 9 closely-spaced cutouts. The door may need swapping (or 10 seconds with a dremel!) if converting from a MK2.5/MK4 to MK5/MK6.