Originally Posted by Lazyass

All you need is a 4.5mm spacer to use a 7 speed cassette on a 8/9/10 speed hub. I ran a set of wheels for 10 years like that. And yes a 130mm hub will fit in a 126mm spacing, even on an aluminum frame.

Cassette Spacer > Components > Drivetrain > Cassette Spacers & Lockrings | Jenson USA