Spacing wouldn't made any differences with friction shifting. As long as the high and low limits can be set to reach far enough to shift into the high and low gears you should be fine in that respect. I'm running a 6sp Shimano 105 derailleur on a 8speed rear end myself with no problems. Get it up on a work stand and see if you can spot the problem. Sounds like the high gear limit screw isn't properly set. After that, I'd say a new chain can't hurt at all.