Mark,
Have you talked to Chris Schimmel? You know he would do a quality job. The only rub is that you would need to get the radiator cleaned out beforehand so that the weld isn't contaminated from the backside. I think Simple Green and some other alkaline solution would clear things out...
Here's a quickie list of stuff for aluminum cleaning (at the end of the page):
http://www.lincolnelectric.com/knowledg ... igalum.asp
A neighbor had an NPT fitting welded into his AL Honda oil pan by a guy out in Durham. I have his address somewhere.
--Ashraf