I know I'm an unrepentant cheap ass; but spend the $830. That's my advice, get 24 lf of 18" CMP (check is 24lf of 15" cmp is significantly cheaper, but i would want too see atleast $200 savings to down size.)
Besides corrosion (your not in a salt water environment, are you; tanic acid from very dark water swamps can attack galv too); the life of a pipe is primarily on installation. Scrape off the muck, place up hill invert at the flow line of creek; down stream 0.1 ft lower; avoid bellies in the pipe; fill to the pipe haunches with clean dry material and compact that; then fill to the top of the pipe; compact that; place atleast 6" of same above pipe; compact, then place atleast 6" of surface material (rock/gravel/millings/base). Locally, we only bed in rock if ground water forces the issue; a #57 stone or other washed rock allows water to flow outside of the pipe, leading to soils eroding and creating voids, and thats not good. On the ends; assuming 100% private property; take some 60# sakcrete bags or soil cement bags, and make a stacked soil cement head wall, to avoid erosion problems.
