I have a Series 5 crank pulley on my Series 4 block, I read somewhere that the timing on S5 motors is ever so slightly different and I was wondering if this made a difference on the timing marks on the S5 vs S4 pulleys.

 

Mounting an S5 pulley on an S4 hub (or vice versa) will put the timing marks in the wrong spot. You need to match the hub and pulley.