I agree about diffusion. Do whatever you can to get a nice, soft light that still gives some shadows. A north-facing bank of windows is always nice. Your 500 watts might be a little weak after you diffuse it. I use a 900 watt soft box very close to my subjects and it's barely enough for 400 film.
I use a Bronica SQ-a as well. I would recommend going for the 150 or 250 lens, which you can pick up for around $100 at KEH.com
You might consider a very light green filter, which evens out skin tones a little.