I have used the Pentax 67 many times for outdoor wedding photos mixing flash and daylight. I do exactly as Steve Smith says using a Vivitar 285 and I set the flash to give a stop or stop and a half under exposure and set the camera correctly for the daylight. I have the 9omm leaf shutter and that gives more options for shutter speed but if you are stuck with the 30th you can use a slowish speed film and make it work.
For a test, regardless of the type of flash metering, I would try setting the flash meter like the film was double the speed it really is and then set the camera ambient meter for the correct film speed. Works for me.

Dennis