David,
Between 6 3/4 and 7 1/2. I wouldn't suspect you'd go out to 7 1/2 but as you know, Dan and Jared's bows are easy to tune. Throw some bare shafts through it. I usually use the same carbons for my equally weighted Whip. 64" Whip and 68" Super D. They bare shaft close enough. Both are 53@30. Brace on my D is 7 and 7 5/8 on the whip. I set the nock height on my Apache tillered D and whip to 5/8 for 3 under shooting. Ymmv of course.
Also, Dan's strings are so light, they should help squeeze as much energy as possible out of those limbs. Love those super D bows. They are proven killers in this house.
Did you get a new super D?