You need a Disc Harrow with Box Frame construction. An angle iron Disc will not give you the weight you need, they are minimally adjustable and in your stoney soil will loosen up quickly.
You will be able to pull a Box Frame Disc Harrow with 18" diameter pans with no problem. Depending on how thick the pans are this will give your from 37 pounds to 40 points bearing on each pan, which is the minimum necessary to be effective. You will be able to set the gang angles as aggressively as you like and still pull the implement at a good soil mixing speed.
Almost all Disc Harrows with 18" pans come with notched (scalloped) pans front and rear to improve cutting.
You can buy a Disc Harrow with either 7-1/2" or 9" spacing between pans. As you are going to moldboard plow I would buy with 7-1/2 inch spacing as it will leave your soil quite level and smooth.
Almost every Disc Harrow manufacturer makes 16/18" (9" spacing) and 20/18" (7-1/2" spacing) in 6' to 8' width. Keep in mind that some spec DHs with gangs flat, to advertise max width, but you will be using with gangs adjusted >< so your working width will be less and will be variable. You will be able to pull an 20/18" eight feet in width if you desire. Land Pride 18" DHs all have 7-1/2" spacing, 72" + 90" (flat?)
I have a Howse 16/18" (587 pounds) for which I paid $1,000 (new) in February 2013. TSC's "Countyline" is currently produced by Tarter in Kentucky. Land Pride is a cut above Howse and Countyline. There are many others. Most have CLAMP PLATE adjustment of the gang angles, which works fine but is slow to adjust.
Everything Attachments in North Carolina has DH with good weight and SCREW ADJUSTMENT of gang angles, which costs more but is convenient. Freight might be considerable to Michigan.
You may want optional (clay) disc scrapers on your DH, or you can rig chain loops between pans to cut off clay accumulation.It is best not to churn wet clay, but sometimes you have to get into the field under less than ideal conditions.
Buy as much weight as you can afford.
From my experience with both 18" and 20" diameter pans, you will not have enough power to pull a disc with 20" diameter pans, which are 50% heavier than DHs with 18" pans.
LINK:
http://www.tractorbynet.com/forums/attachments/308251-disc-harrow-selection-18-45-a.html?highlight=
LINK:
DH15 Series Disc Harrows | Land Pride
LINK:
Everything Attachments XTreme Duty Deluxe Box Frame Disc Harrow