Yes, there are 340 Challenger convertibles in '70 & '71.

1970 Challenger
/6
318
340 264 built for USA
383 2bbl
383 4bbl

1970 Challenger R/T
383 4bbl
440 4bbl
440 3x2bbl
426 HEMI

1971 Challenger
/6
318
340
383 2bbl
383 4bbl