Did you know the difference between the Borg Warner M-11 (290, 343 & '70-'71 360) and M-12 (390 & 71 401) automatics?
The M-12 is basically an M-11 with a larger clutch pack. And therefore holds one less quart of tranny fluid. Which means if you use a dipstick from an M-11 in a M-12 it'll show your a quart low on fluid when your not. Fill it to the full mark and your tranny is one quart over filled and will blow fluid out the dipstick tube.
Put a M-12 dipstick in an M-11 and it'll show it's full when it's a quart low - not a good thing.