My research showed the 340 and 383 where available in the 1969 Swingers and GTS models, the 383 weren't available in the GT models in 1969 shruggy
They, 340 and 383, were, supposedly, available in the GT models in 1967 and Swingers and GT in1968 confused shruggy

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)