Hi, I picked up a Mirage long cage for not a lot, almost new condition. I does not look like a "top of the line" construction. My mechanic wants me to get a Centaur. But he wants a long cage. He wants to make sure we do not run into a problem with chain take up. How hard would it be for me to put a long cage (assuming I find those parts) on one of these Centaurs if it is purchased with a short cage? I found a short cage Centaur for sale, I think it was from a seller in Europe.

Yes, it appears from my research that indeed the Euclid was for a mountain bike. My mechanic told me a mountain bike derailleur could be used. I asked him because the usual find on eBay for a long cage used derailleur seems to be mountain bike specific derailleurs, I am assuming because of the gear range for hills.

I can't answer your question about the shifters. All I know at this point is my mechanic told me he can get trigger shifters (which is what I told him I wanted on the bike) that will work with a Campy 8 speed rear derailleur. He didn't say what he would be getting. I don't much care if he can make the whole thing work.