There are only two foolproof ways of telling if the trans is an overdrive
1) pop the cover and look; this is foolproof, or
2) put it in third and compare the input turns to the output turns; same in/out is an od box
And several other ways that all these years later, may or may not point to an od-box;
3) the od has a 5.125 front retainer and none other does. But this ain't fool proof, cuz someone could have swapped a standard gearset into the od box.
4) the od boxs usually are aluminum. the standard usually are cast iron.
5) the A-body od-boxs are usually 904 output sized
6) if a box for a 340 or bigger, it will have a 727 sized output
7) there is a PN stamped on the RH side.Just look it up.
8) Shifters and levers and orientation,may or may not be helpful.
9) if it has a long tail, with two shifter bases on it, it was never born an od, unless maybe it has a Chevy input. But then it will have the 5.125 rerainer.
10) There may be more..........
that's the best I got,
except for a streeter with a performance engine, the od box rarely works out, because the splits are just to wide.
Oh yeah, and IMO, a ball-shift handle is the best; you have so many options in the way you can palm it. and rarely might you slip off it, and it's really hard to bust your knuckles when you do.
