Do you have any wider lenses for the Mamiya than 110mm? Something like 80mm or less? I ask because 110mm is a bit on the long side for street photography in an urban area with medium format cameras.
The Speed Graphic is a perfectly capable street photographer's rig and no tripod is needed. Handheld is fine.
I just did some street work with a Mamiya C330 yesterday, prism finder & 80mm lens. This was my first time using a prism finder on the street (usually I use a chimney finder). The upshot is I was composing images a lot more quickly and accurately, and they weren't at chest-level. The downside is I lost the element of surprise. I didn't use my 65mm lens because the bellow extensions usually require too much exposure compensation compared to the 80mm, which rarely requires any.