Notched in hot rodding (to me) is - if you can imagine a C "opening" is notched in the bottom of the frame so the frame doesn't interfere with the axle when the chassis is dropped down low...
Narrowing or PINCHING, to me, is to make it narrower -- in the hot rodding sense. "Pinching" usually is done on '32 rails to fit the narrower '29 cowl... so if a guy told me he pinched his frame - that's what would come to mind.... if he narrowed it - and mentioned the rear end - I'd think he's running "pro street" style... and if someone told me they notched their frame - I would imagine them running airbags or the car sitting real low without being "Z'd" (another term of frame mods).
I think - at least to me - that maybe explains why people aren't or weren't sure what you were asking about???
That -- or I've been reading way too many hot rod magazines...