I recently did the open cooling conversion and eliminated the old tank, replacing with an expansion tank from the 2.5. It's a super clean install but I had a hard time finding a cap for the new tank. I'm sure they're out there but my googling didn't find them.
If anyone needs to know, the cap is a 48-400 size. I had to order a pack of 72 so if anyone needs one, send me a message and I'll mail one out, I have plenty of extra.