Even though I’ve never actually seen TGTBATU I’ve heard the original score often enough, dry empty and ragged where Montenegro’s is polished and full of incident. In the days before film soundtrack releases the shops were full of “…and His Orchestra” cash-ins, usually themes themed along Western, Spy, Love Song lines. Montenegro, with a pedigree in space-age exotica, was probably at the classy end of this spectrum but it would be easy to criticise his version for perceived inauthenticity.
Easy yes, fair no. The purpose of a film score and a pop version of a theme are very different: the former is setting a scene, helping you go in deep to a set experience. With the latter, the scene it’s setting is your own listening space – a living or bedroom in 1968 most likely. Sure you want to be reminded of the film, but maybe you also want to be in the film. So Montenegro’s smoothing out of the tune, his poppification of it, his greater emphasis on the beat and his nonsense vocals are partly tactics to make it easier for you to involve yourself, strike a pose like Clint.