A less expensive approach to the cassette issue would be to buy an 8-speed cassette and just take one cog and spacer out. Then you would have 8sp spacing. You might need a little shim between the largest cog and hub in order to get the lockring snug enough to lock the cogs down. If you get the cassette from Harris Cyclery and tell them what you are doing they can tell you what you need. As someone mentioned the 7-speed freehub body is shorter than the 8-9 sp body (same body for 8 or 9). Are you sure you have a freehub/cassette arrangement vs a 7-sp freewheel? If you are not sure of the difference, refer to Sheldon Brown's articles. If you have a freewheel, you can just thread on an 8-sp instead but you would probably need the longer axle. If you have serviced you own hub bearings, changing the axle is no big deal.

There are often (NOS) new old stock 8-sp STI levers for sale on eBay.
Regards,
Raymond