Are both 3.8L & LS4 using the same bellhousing pattern? Because apparently the 6T70/5 should bolt up; I've been on the net a little more, and there are folks doing LS2/4 & 6T75 adaptations, as well as F40! One bloke put that combo in his Fiero! But I'm reading that as long as the Metric bolt pattern's there, it should​ work... though the starter for 5.3L might need an adapter...