I would say you should try it both ways.
You'll need to set up an impromptu drag strip on your street. Get some of the neighborhood kids to act as the signal tree and time you with a stop watch.
Which ever way is fastest is the right way.
The seat bracket may not be big enough to spread over the bolt on hard tail part. Mine runs a coupling nut welded across the back of the part where the back bone turns down and then bolts through that. Seen quite a few like this. Not as clean, but may be what you are after.