If you want the height, your 8' spacing is fine for untrimmed plants. Because of the conical shape you'll have lots of space between them for the first 20-30+ years. If you want a screened hedge, it is fine to plant them 5'-6' apart and maintain them to insure full growth down low. You can plant another row staggered only 3'-4' from the first row, it doesn't have to be 8' away. If they were small potted and you just planted, it should be easy to relocate if you choose so.