If you have money to burn (approx $1800) you can get the "g-Bar Canted-4-Bar Suspension System" from Chris Alston Chassisworks (
Chris Alston's Chassisworks). The advantage of this one is that it does not have a panhard bar, and it has its own subframe. So you order one for a '65 Mustang, bolt the lower arms to the MII front leaf spring mounts, and weld or cut as necessary to attach the subframe to the bottom of your MII. If the front halves of the MII leaf springs are a different length from that of a 1965 Mustang, you may have to shorten or lengthen the lower arm a bit, but that shouldn't be to hard. Your mileage may vary!
A cheaper possibility that also skips the panhard bar is this triangulated 4 link kit from Paul Horton's Welder Series (
Welder Series Online Store - Street Rod and Hot Rod Parts Kits Ready-to-Weld) for $325. The problem with this one is that you have to do a lot more work to determine mounting points.
I think that any solution that drops the panhard bar is really going to simplify your fuel tank and exhaust issues.
Ah, one more thing...400 HP + Mustang II T-Top = welded subframe connectors.
Let us know how the project goes!