Whatever you do, don't attempt to cold set (permanently bend) the rear triangle of an aluminum frame. That's for steel frames only.

Regarding your statement that you only need eight gears, and so you don't need "all that room back there" (i.e., the 135mm OLD hub), I'll point out that the freehub on modern Shimano-compatible 130mm or 135mm hubs will accept the same 8/9/10 speed cassettes, "road" or "mountain.". The wider 135mm hubs are "mountain" hubs, less dish makes for stronger wheels. Has nothing to do with which modern Shimano-compatible cassettes it will accept, though.