The slant or the 318

I have a 1972 Duster with a 225 and a factory 3 speed on the floor. Car runs great! BUT I have 318 with 6k on it.
I am toying with a slant 6 but it seems expensive compared to a V8 swap! The Duster runs perfect and is fun to drive just kinda gutless and the 3 speed seems to have all the wrong ratios.