Which letter will replace the question mark (?) in the following series? M9, R64, W25, B216, ?

F36 G49 H64 G343

G49

According to the above alphabet position: M9    ⇒ (13), 32 R64   ⇒ (18), 43 W25  ⇒ (23), 52 B216  ⇒ (2), 63 ?       ⇒ (7), 72 = G49 (add 5 in 1st alphabet position and there is square and cube of consecutive numbers respectively in 2nd alphabet position)