According to comments in the database, the legacy Pentax flashes that you mentioned should work okay:
Flashes (Genuine Pentax - Legacy) - Pentax Camera Accessory Review Database
But to be sure, the trigger voltage is very easy to check. Just put a digital voltmeter/multimeter on the center pin and the ground pin.
I have a Wein Safe-Sync which is very popular/common. Some PF members have pointed out that this gizmo is kinda not worth it, because it costs about $40 and you can buy some newer flash units that don't have TTL, but do have more power, more manual settings, more auto settings, etc. for not too much more money. I was in a similar situation as you, with a small pile of old flash units, and I didn't want them to go to waste, so I got the Safe-Sync and use it all the time. To me it was worth it.
Another idea. You can also buy some cheap optical triggers that mount on the flash hotshoe, for off-camera flash. I bought a couple Adorama house-brand units for like $12 each. They mount on the hotshoe and will fire the unit in response to another flash. Most of those are rated to handle a high-voltage flash. Mixing P-TTL and optical slaves doesn't work well but if you are shooting in full manual, then having some extra optical triggers can be fun. If you do decide to get some optical triggers, I suggest getting the cheapest ones you can find, because a dedicated optical slave flash can be found for pretty cheap as well.