depends on the year:
70 R/T 383 4 bbl, 440-4 bbl, 440-6 pack, 426 hemi...340 cars are not R/T's in 1970
71 R/T 340 4 bbl, 383 4bbl, 440-6 pack, 426 hemi...No 440-4 bbl R/T's in 71, and NO ragtop R/T's either, all 71 Challenger ragtops were JH27's...largest engine available was a 383 4 bbl in a ragtop 71.
NO R/t's after 71..they were called rallyes from 72 on..and largest motor was a 360 in 74...340's in 72 and 73.