Clearance in which direction(s) are you concerned about?
I think space between PCI slots - to have a better gap between cards
That's why I asked to make sure... I assumed that the spacing of the edge connectors was a standard for an ATX board, so any board will fit in any case.
The spaces between the slots is ATX standarised to a tight tolerance.
Some boards will leave out some of the slots, but the ones that are there are still on the same spacings relative to the screw holes and position from the board edges.
Board widths are also standardised within a few varients - ATX, miniATX, microITX, extendedATX probably being the most common varients.
The depth of the boards CAN vary a fair bit, the ATX standard was somewhat loose on that part of the spec, and that CAN in rare cases cause board fit issues on some small cases and deeper boards.