You will likely need a new Bell housing as well. The overdrive 4 speeds all have the large (5.125 I think) bearing retainer. You may be able to machine your current bellhousing to fit.
"Back when" and "way before" the internet, when we swapped 340 into what-had-been a 70 RR / 18 spline / sixpack, that is exactly what we did. Anyone with a large enough lathe can do so.