pstock,
You can't just put the 8-speed cassette on the 7-speed HF07 freehub since it's not long enough. You'd have to swap over the freehub from FH-M756 as well. If you do this you will have to adjust your axle spacing and redish the wheel to account for the longer freehub. Sheldon Brown has a blurb on this
here
I did just this on my '92 Trek T200. I put a new 8/9/10-speed freehub on my old 7-speed Shimano 40-hole hub with new Dyad rims and an XT 9-speed cassette. Works great!