I also agree on the use of a good quality black poly pipe. Nothing less than 160 psi. I have dug up lots of 100 psi pipe for pin hole leaks, cracks. also use brass fittings from the poly pipe to the hydrant, I wouldn't use galvanized, or plastic fittings. same reason, dug up many of them for repair, replacement. galvanized will rot away and plastic will break or just start leaking.
I know what you are thinking, the outer pipe on hydrants are galvanized, yes and I dig them up all the time when they rot and replace them.
Also use all stainless steel hose clamps, two for each fitting, a lot of the cheaper hardware store ones look stainless but the tightening bolt,screw isn't and will rot away over time. Jim
Now this is for an 8 foot bury line, you didnt say how deep you will be with the line?