In an E Body the driveshaft is the same length between 4 speed and 727, 904 takes a longer shaft than the 727 with a corresponding slip yolk made for the smaller output shaft.

Using an aftermarket cable shifter makes the swap easier by avoiding the need for a column change or the mounting brackets for the shifter.

Cooler lines are not a issue - purchase them or bend your own.

Using a late style 998/999 (some guys call it a 904 - 998/999 has wider front bands and more clutch discs) gets you the 2.7 low gear set. It can easily handle the power of most 340’s if properly built.