I can't imagine a 480 horse 340 "sounding" like a sleeper. ha ha.
My idea of a sleeper is first, MUST be a moredoor. Far too many "cheap post" bodies were sleeper wannabees. Wagons work.
Second HAS to be stone dead quiet. If ya need 4 mufflers, then, ya.
Of course, NO headers. The telltale tinkedy tink. Maybe wrapped ones'd get buy, if you can't SEE them hangin.
No "wheels" of any kind, and if you run so much HP that you need BIG rear meats, then you screwed the sleeper image. Might just have to be happy with "rolling" starts.
No visible tach, and no big autometer, shift lights, "boxes", etc.
No fancy floor shift, B&M or Cheetah shifters
No visible trunk mount battery disconnect
And of course YOU have to "be" the part. Look a little nerdy. Suck it up and USE those flip-ups. Wear a silly hat. Kids toys in the rear window. Maybe an empty baby bottle, er empty kids seat in the back.