Question about cropping in negative space on Aperture

I have a question about cropping on aperture. I would like to print a photo that is 6 x 10 inches. No photo company prints this size, so I was going to print 8 x 10 and physical cut out the extra space. Unfortunately my photo is already zoomed in and i can't figure out a way to crop and include negative space. Is there a way to do it on aperture. Should I use a different software program. And when i print to Wallgreens or shutterfly, they just strech or crop the image even further. Sould I be using a better printing company?

Thanks!