The difference in A-frame length's are due to the mount styles... a conventional mount uses a longer A-frame than an EZ Classic style mount since the conventional was further back under the truck.
But with a chop saw , torch and welder you can easily change them over
The C models use pretty much the same a-frame and quadrant from what I have experienced the ST models use a different a-frame and quadrant from the c-models. The mounting ears on newer plows typically have the same spread, its the 20+ year old ones seem to be almost vehicle specific. Like I mentioned before with the right tools you can easily adapt them. The last 7.5' ST MDII I sold had a very rough blade attached to it, I found a real nice sold and straight ST 90 locally, it came off of a early 80's wagoneer, after a sandblasting and couple hours later it looked like and functioned like it was always a MDII.