904 won't do anything

The 18 to 27 spline conversion can be done if you have a 67 & later 904 as a donor. You need obviously the input shaft & forward clutch drum assembly, the reverse/high clutch drum & the reaction shaft support from the donor transmission. With any luck the donor transmission will have the snap ring groove in the rev/high clutch drum cut higher to enable 5 frictions to be fitted.