You will also need a different frame member that supports the Xmission, and a different emergency brake cable. Then there's the speedometer cable, which is also different. The driveshaft also will need replacement, as I recall, and the accelerator linkage is different. OOps- the differential ratio is different for hydro, so one ideally would replace that as well. The hydro has a bell housing that is different, so that would have to be acquired along with the Xmission.
I'm sure this is not an exhaustive list, but will give you some idea of what you will have to be doing. Getting a parts car with a hydro would save you lots of searching.