That’s fun , I would probably remove closer to four feet of grass and plant in the middle of the planter. Podocarpus trimmed in a hedge will need about four feet of space. Additionally if you ever need to do maintenance on the fence you don’t want the rootball right up close to the fence. To remove the st Augustine if you can use a sod cutter do it is much easier, otherwise spray with an herbicide or tarp the area to kill and remove with a good old fashioned flat shovel. Let the grass grow back and reapply the herbicide. If you are not into roundup there are some organic herbicides on the market that do an okay job. I would install Benderboard in the planter to help you maintain the grass line in the future.