To me, your welds look good with good penetration. You can get tubing to route argon into the fame tubing if desired, but I'm not sure it is necessary.

What size is the seat post? I could probably locate a 27.2 quill post for a reasonable price.

Seatpost.jpg

Oops, it looks like the retention mechanism will limit how far you can drop the post

So, your actual gains may not be much more than simply trimming the top off of the mast.

Assuming that is some kind of a cam lock that you have, you could probably pretty easily move the cam lock to the under side of the top tube, or seat stays, and braze it into place.

In the end, you'll have a most unique looking bike