trying to picture your question here?

If your rear gearing is that small. You have a Road derailleur and not Short cage.

This will only allow you to go to 26index or 28 friction. It will grind into a 28t.
You can lower the front to a 39, but won't find much of a diff.

Won't low? Get a GT or Long cage older shimano 105 allows up to a 30t

Ok a Cluster isn't the same as a cassette. Yes a 6sp cassette only works with two chain whips to take the outer one off. A Cluster is also called a Freewheel and is screwed onto a hub and uses a Freewheel tool. If you are staying with INDEX it's very important to stay the same. "BUT IN FRICTION - you can use any combo you want!"