Why do you want to install a prefab shower unit?
Why not have a plumber install a shower pan (I like the pvc type material over the lead), install the shower controls and heads where you want them, then have a contractor install a suitable tile substrate (DuraRock, WonderBoard, etc.), then have a tile setter install the tile. This will give you a custom shower to exactly fit the space you have available.
Or, you could compromise and have a custom cultured marble shower base and wall panels made in the color and pattern of your choice to fit your exact space. Installation is easier than that described above and, if installed properly, is less likely to leak at the shower pan. These also look much nicer than the prefab units.