I'm trying to put an 8 speed wheel on my 6 speed Nishiki, so far everything is good other then I have not been able to get high gear combinations to work properly. Could this be due to I'm using the old 6 speed chain ? WHat can be done for it to work in high gears without skipping ?

A little more information may help. Derailleurs, what shifters, etc. I'd recommend picking up an SRAM PC-68 chain (or the PC-58 its a few bucks cheaper). I've got them on all of my bikes (6,7 & 8 speeds) and they work great. New chain is often a good place to start, but more information may help pinpoint other issues.

It has a Suntour Cyclone MK II rear derailleur, Suntour Superbe friction shifters, and Im using a Shimano 8 speed cassette. I was wondering if the spacing could be off ?

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.

there is not enough throw on the MK II (at least there wasn't on mine). which sucks because these are very cool and light!