If I'm in the mood for 8 gears, I'll either put my Ultegra 8-speed wheel on it, or I'll get my Cronometro wheel with Shimano 8-speed cassette repaired and put it back on. I've used the Campy 8-speed setup described above w/ Shimano 8-speed cassette since the Fall of 1995 (when I got the Cronometro wheel). It's worked fine for almost 10 years, I see no reason why it won't work another 10. The only reason I am not still using it is because I broke a spoke and noticed cracks at the spoke holes, so I'm getting the wheel repaired. In the meantime, I'm using my Campy Ypsilon rim w/ Mavic hub and Sachs Aris Shimano-spaced 7-speed freewheel. My wife's bike currently has an Ultegra 8-speed cassette, old 105 RD (the blue one), and is shifted by a Deore 7-speed indexed thumbshifter, and she has access to all 8 gears.
My point with all of the above off-topic items is that sometimes things will work, even though people say they shouldn't, and that we won't know if we don't try. Toddorado should try putting his 7-speed wheel on his bike and adjust his RD accordingly. The absolute worst thing that could happen (barring mechanical errors on his part when adjusting it), is that it won't shift as nicely as he'd like. So he then goes out and gets a new cassette/chain, or tries something else. No harm was done. And if it does work to his satisfaction, he won't have to spend any money at all to get his bike in the condition he likes.
As for adjusting the RD for the 7-speed cassette, there are two ways I'd do it, depending on what result I'd be looking for.
1) shift to the 4th (middle) cog of the cassette, and tune it perfect to that cog. When you shift the range, you'll only be .6mm off at either end.
2) tune it to the cog you ride in most often. The most the shifting will be off is 1.2mm, and that's only if it's tuned perfectly to either the lowest or highest gear. I see about 2mm of side-to-side play of the chain on the cog on my wife's 8-speed cassette/chain, I don't think it'll be a problem.