Here are the firewalls. On the standard, the crossmember is one piece riveted to the frame. On the hydramatic it un-bolts. Notice the "hump" cut out of the hydromatic firewall. The neutral safety switch is located on the steering column.
Here is a view of the shifter in the hydramatic. The key switch is different too. It has a start position.
As you can see, the change over to a hydramatic is not just a direct swap. If you want it to look "factory" you'll have a job ahead of you. But it can be done and it doesn't have to look "factory" if you don't want it to. It's your truck.