Request for help with lower gearing for my 7-speed

CandyEpic

I'm preparing for a cycling tour this summer which will involve a number of difficult climbs. I've recently started training for this event doing a lot of climbs where I live. Generally, it's flat. I learned quickly that I'll have to do something with the cassette to get lower gearing (39T up front, 24T in back). I would like to get a cassette with at least a 28T in the back, preferably a 30T.

Not knowing much about history of freehubs and cassettes, I have been told by a local bike shop that my bike has an old Shimano freehub/cassette style that was only made for 1 or 2 years. Here's my setup:

7-speed Shimano 600 drivertrain
Shimano Dura-Ace rear (free)hub
The smallest cog on the cassette threads onto the outer portion of the freehub.

There is the problem. The freehub I've got will not accept 7-speed Shimano cassettes sold by the likes of Nashbar and Performance. Aside from looking on eBay for parts, what can I do?

I am hoping someone with knowledge on this subject can help me out with following questions:

- which years were this style of hubs/cassettes sold?

- Are the spacers and cogs for Shimano 7-speed cassettes and 8-speed cassettes of the same era interchangeable?

- Can I use the larger cogs from an 8-speed XT cassette along with two of the smallest cogs on my freehub as this article suggests?
(see link below)

- Does slight twist on each tooth of the cassette serve a purpose other than "filling" the slot in each chain link? I ask because it appears that (looking from the top of the bike down) the XT teeth are straight, while the teeth on my 7-speed cassette are slightly twisted.

I really appreciate your thoughtful replies. My screaming knees will thank you also!

www.insidetri.com/train/bike/articles/1328.0.html