from a technical standpoint, i'm going to say NO NO NO! use your clutch when you shift. ok, here's why.
now, i have no experiance with american muscle what so ever. but i'm under the impression mustangs dont normally have syncronizers? you may not shift with the clutch on your mustang, but if i'm right, the gears are tooth'd different. they dont cut diagional sideways like \ but rather just -- like dog teeth gears.
regardless. if you want to improve on your 60ft, you would still want to use the clutch. honda's have syncronizers in them that allow you to put it in a gear when RPMs are not matched. so what you're looking to do, is shift very quickly. aka powershift. use the clutch, but shift fast as a mothafucker. then, the RPMs of your motor won't have enough time to fall to the same RPMs as the next gear, so when you return your foot to the floor on the gas pedal and release the clutch, the power and drag from the motor and your clutch's holding power will pull the driveline side (trans side) up in RPM, if you see what i'm saying. this will make your car jump about 12mph in a 1/2 second (edit, from 1st to 2nd). dont belive me? i've got data logging to prove it.
that's another reason for you to use the clutch, not to mention hurting your diagionally cut gears...