Yes, but retain control of the die fabrication. I do not believe OK is currently paying as much attention to accuracynof the extrusion as you all would like.
Since my "last post read" button is not working real well this AM, I just read the entire thread quickly. The real solution is to find this guy who had the Williams extrusions done in 80' lengths, or even to contact Bachmann and see if they will do another run. The expensive part is the die - not bad, because they now use EDM (whatever that is). But Jerry already did it to Pete's satisfaction, which, if I wanted Amfleet, would be good enough for me.
Cutting windows yourself is not something you want to tackle, unless you have a good size end mill. Once you figure out the extrusions, you might want to talk to Joe Foehrkolb about machining.
And for ends, I bet Williams will sell them for a price. You guys have an interesting project ahead of you.