Originally Posted by guy153

If it's external butted you can just TIG weld right onto that. That's basically why they do that. Obviously make sure you cut any extra tube you don't need off the bottom so you're keeping the fat bit that you need

Then you can just leave a bit of ST sticking up above the TT and stays so you can cut a slot and either just buy a collar to clamp the seat-post or weld on some kind of a pinch-bolt. I leave 48mm of extension which works well but you probably don't need that much.