Slant 6 doesn’t go back in

It appears the 1970 Duster had a 1966-67 engine and matching transmission. Try reading the casting# on the block and compare to a list floating around on this site.

Haven't done it, but re your question of using the red torque converter (smaller snout, 1967-) in the later transmission (1968+), I have read that can be done only if you replace the 1st assembly of transmission guts (input shaft, maybe 1st drum, maybe oil pump). I think it doesn't slide in because the #splines on the input shaft differ.

It might be easier to get an earlier transmission. If so, I think you need 1966-68, since 1965 had a cable shift and different output. The speedometer cables might also vary. Problem is I rarely see 1960-80 Mopar today in any junkyards in California. Even my 1985 M-B 300D is getting rare. Indeed, I am visiting SoCal currently and was looking for parts while here. Looks like Ecology was bought by LKQ. They have a web search, but you must search each of their yards individually. In NorCal, the row52 site searches all yards, which are mostly owned by PicNPull (Snhietzer Steel) now.