I have a 80GB 5.5gen now and am considering to buy a stock 7gen and upgrade it to 240GB (and use 5.5gen as spare). My plan is to buy a bare 240GB drive without addtional back cover, put 80GB 5.5gen's back cover and 240GB onto 7gen, then put 7gen's back cover and hard drive onto 5.5gen.
This is because I like 7gen's sound better than 5.5gen. 7gen can still be rockboxed, though not as easy as RB'ing a 5.5gen.
Make sure the 5.5G's back plate fits the 7G's front plate. IIRC, the 5G uses plastic clips on the front plate to attach to the back plate, while the 7G's back plate uses metal clips to attach to the front plate.
You should also make sure the cuts for the lock switch, dock, and headphone jack line up perfectly, too.