Routing a HV Pulse onto a PCB
tl; dr: air gaps on PCBs are lousy HV insulators. That's what you're proposing by using PCB spacing.
Even if you find a clearance spec for your given PCB material, it’s difficult to control the performance of it using normal PCB manufacture. That's because you're relying on air as an insulator, which is influenced by environment (humidity, altitude) and the PCB insulation strength.
More here: http://pcb.iconnect007.media/index.php/article/53785/pcb101-fabricating-high-voltage-boards/53788/
What to do? Besides choosing an adequate board material, you can consider potting the traces on the board to achieve a strong enough dielectric. That can work, and it’s a technique used for things like CCFL drives that have multi-kV strike voltages.
If you're contemplating a long HV trace... that's a lot of of potting material. By the time you consider all that special process work needed, what you may discover is that it'll be more cost- and area-effective to use wires with adequate insulation strength, and then contain the board's HV problem to only the point from where you're launching your pulses.