Your drag advice needed

My best advice is to pull the rear end right outta there , insert your mounted wheel tires assemblies into the tub and center/square them up. Then measure between the mounting faces of the wheels and get your diff narrowed as required.
It is better to be a half inch too narrow than a half inch too wide.
Tuck them as tight to the frame as you can, leaving a minimum .25 on the inboard sides to the nearest obstruction,which at lowered height, after the front curve in the tub is stretched away, will be the shock-crossmember. Forget trying to pound that away, it don't budge much.
On the outboard side is the stiffner step in the inner tub up by the top of the quarter lip. It's about 1.5 to 2 inches higher than the lip. Your 275 on a 10 should clear it, but you never know. A 295 on the 10 won't. But you only need about an inch vertical clearance and then it is no longer an issue. .... it just depends on how low you wanna go.
I know you didn't ask, but you're the first guy I ever heard of who liked his Duster low. Not sure what the Dragstrip starter box is gonna do with that, and the 2" lowering blocks, and the driveshaft sliding back as the springs torque up.
If you lower the front a like amount with the T-bar adjusters, yur going to get into toe changes with camber changes, as the hood goes up and down. And that makes the car steer funny. If this is to be a DD or yur gonna put a lotta street miles use on her, you may end up chasing the car from one side of the lane to the other, on anything but flat and smooth roads. And it won't matter what you set the toe to, it's just gonna have a mind of it's own.
I'm just saying there is a too-low place you may not want to be in; this ain't no 64 Impala.