From all I've heard you cannot weld cross link no matter how good you are...
However, my old Y I am pretty sure is cross linked and I welded it... it's still holding though I have welded different cracks near the original 3 times...
I am actually starting to think that if the crack isn't too bad, just drill the ends of the crack.... then 3-M vinyl Mastic on the inside... The stuff sticks to anything and remains flexible.
http://cgi.ebay.com/2-ROLLS-NEW-3M-SCOTCH-SEAL-2229-COMPOUND-3-75-X-10FT-/280501564844?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item414f3235acIt's 3M 2228 (in case this link ever dies and somebody wants to google it)
It's expensive, but I have two rolls of it and would sell a bit of it if anybody is interested...
So far my patch is holding great like that, I like it much better than my old tons of duct tape method...
Good luck.