Rear disc conversion options

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Ive used disc conversions from right stuff on a 10 bolt 8.5, all were stock 11” rotors, Is there a larger disc conversion avail. (12 or 13”) without going to an expensive baer or whatever expensive brand? The right stuff kit is around $400.
Gonna run a 18-20” rear wheel, want something to fill the space if possible..

Will also be looking for a front 2” drop spindle for mustang ii with gm rotors, pretty sure no limit has something, but looking for options