Good Morning, Karl,
There should be zillions of flash units that would work just fine. Almost any modern unit will have an auto-exposure mode that enables it to work with any camera it can be connected to. I've had good luck with Vivitars, but there are lots of other possibilities. I've never used a Pentax 6 x 7, but I think that the sync speed is rather low, maybe 1/30. That may cause problems in relatively bright light, but the problem would be with the shutter speed, not the flash.