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

Well, you made a decision at least. In my mind how long will it be before that 5/16 is 1/2 or more? Probably not long. Good luck!
Double roller chain. Being a Chrysler authorised reman it is likely not a top name timing set, but I do not think another 100k miles will wear that timing set much more. My concern would be the cam and lifters being used with modern oils that do not protect the cam and lifters from premature wear.