I'm sorry but I can't recall at the moment which leadpipe it is... it was about three or four years back that I read about the conversion. I know that a guy named "Albert Lilly" posted quite a bit about it... he recommended it as a method of turning an Ambassador into an excellent, pro-quality horn (improving on what is already pretty good).
You might send him an email... I'm sure he'd respond. Here's his email:
alilly@scican.net
For the cost, I wouldn't imagine it would take over about $150 or so but you'd need to talk to your local repairman to be certain.