Sounds like you have a series 70 or 80 plow. I think this is so because you mentioned the lack of clearence and the 375 bottom. However your description of the moldboard supports sounds like a 50 series plow. Not sure if we are talking about
the same thing. You said the support was "straight" does that mean the support is mounted straight up and down relative to the ground or is the support itself formed
"straight" as opposed to curved?. A picture would be great-I could tell you for sure in a second. The 70/80 plows were meant for the D
series tractors, although you could use them on a
WD. But clearence could be a problem depending on your tire size.

Hope I helped
Tom