i doubt stock (8.7" ) will be too long , with roller lifters they fall around 8.5" and roller lifters in general require a push rod .5-.6" shorter than with flat tappet .
you're probably looking at a 9" rod for flat tappet on alum gn1s but youll need to measure ,
with std height valves should fall in the range Tom suggests , he did mine with 1/8" taller valves