I just got done making a new pump drive setup on my 3414.
As to your's - it all depends on the serial number and which pump it has.
Early 3414 has a pump with a round shaft and single key.
Later 3414 has a 7/8" X 13 splined shaft just like many Deere, Fords, Cases, etc. use.
On mine - the pump had the 7/8" X 13 splined shaft. Then a 7/8" X 13 splined coupler that you can buy on this Website as a Ford part.
From that coupler - the shaft is 7/8" X 13 splined on one end - BUT tapers down and is only 3/4" splined on the other where it connnects to the drive plate on the crank-pulley.
If your coupler is bad, it's likely the shaft and drive-plate on the crank pulley is also bad. I adapted all new parts that I bought here. All for a Ford loader. New rubber-cushioned drive plate with a 7/8" X 13 hole instead of 3/4". New 7/8" splined shaft, new 7/8" splined coupler.
If ALL you want is a coupler - you can buy here under Ford. 7/8" X 13 internal splines.