Hey all,
I was building a Colnago Super (don't know what year, but my uneducated guess would be late 70's) and I came across a problem when setting up the rear derailleur. The housing stop on the top of the chain stay is too narrow for the housing to fit into, and I tried three different types of tapered farrules and the closest fit was a cruiser brake one but it didn't fit properly. Any ideas on the hardware I need to finish this? I'm pretty new to bike building so any advice is appreciated.

It would be interesting to see a picture. There was a kind of stop that was built to take a stepped ferrule, you might have one of those

I think you are looking for this:
It's indeed a step-down ferrule that slips on the housing. You could also use a complete stainless steel cable casing.
Both seem to be a little hard to find in regular shops and online.

Yup that's exactly what I'm looking for. I guess I'll just have to hunt for it. I'd post pics but the bike isn't at my house, so it'll be a few days. Thanks though.

You probably need something like this...
https://cgi.ebay.com/Campagnolo-622-R...-/110686603379

Bicycle Classics has them as well, but not sure if they will reopen?

$34?

I think I need the same thing for my Gran Course. It was working fine until the core pulled through the cable housing and slightly tweaked the cable stop. Now nothing fits in there properly, so now my bike just hangs in a corner