Since he's already been running the 11.65 rear rotors, shouldn't he already have the hubcentric rings for them? Or are the ones he has old enough to be Thunderbird specific that are no longer made?
Long story short OP, the rear rotors can have the lug holes made oblong, so long as you have a ring to put between the hub of your car and the inner race of the rotor to fill the space. Same as if you bought aftermarket wheels, you should get hubcentric rings to fill the space so that the wheel is centered on the hub, instead of by the bolts. Otherwise, if you don't have those you can end up with issues that mimic wheel imbalance.
97 Thunderbird LX (The GT Bird)
2003 Explorer WAP block with Modular Head Shop "street ported" heads and Stage 2 PI NA cams, 75mm Accufab throttle body, C&L upper intake plenum, Kooks 1.75" primary/3" collector headers, 2.5" full exhaust with mid mount Magnaflow dual in/out muffler, 24lb/hr injectors, 80mm MAF, Tuning from Don @ www.lasotaracing.com
, CAI that feeds from fenderwell. Jmod, 3.73:1 TL in Mark VIII carrier, Mark VIII aluminum LCAs, 93 Mark VIII driveshaft, PBR brakes (soon to be Cobras), 18x9 wheels with 35mm offset, 255/40/ZR18 Tires, Front and Rear strut/shock bracing, GR-2 shocks, Eibach 1.5" springs, 1989 SC front and rear sway bars.