What's your opinion on the slack in this 318 timing chain?

It depends if you are going to put the miles on it replace it if its a strip car how many more runs till ya tear it down etc. If one is really worried about timing accuracy go to a gear drive. The chains work and worked well for years...they aren't supposed to be zero slack tight either...that causes other problems.
Old Vauxhall Epidemics would wear the chain so bad you could hear it slapping on the timing cover. Eventually they would wear a hole in the timing cover and leak oil. By that time it was overhaul time. Silly things ran forever like that.