Yes, 1971 & 318 & 4-spd is possible in a Barracuda convertible - the '71 production numbers are a bit messed up, but in '70 ~58 were built (US spec.). The lack of a VIN# on the one with the N96 would lead me to believe that those are visual clues to ID it if further info became available.

Here's a '70 Barracuda convertible factory pink, 318, 4-spd, A/C, & AM/FM.

pic "stolen" from pantherpink.com