Most pole barns around here have grade boards that are already leveled and at the height needed. We just use those as forms
I actually built this building when I was 15 and while it's structurally sound, it's pretty bare bones. I'll try to post a link to some pics today.
Have poured many pole barn floors. Used the metal sheating itself as the form. If needed, you can stiffn the sheeting reom the outside. Get a laser setup and shoot your finished concrete grade. Mark it on the back wall. In 27' you want around 2-3" drop to the front. Trust me on this. Mark that. Now snap a line across your marks on the metal walls. Go over this chaulk line with a permanent marker. Now do all your compaction and fill. Next you need to run visqueen up the walls about 4' duct tape it in place. The visqueen keeps splatter off the walls but moreso keeps your concrete pad off the sheeting so it doesn't rust through prematurely. I would softcut 2 joints across the pad in both directions for expansion joints the next day. Simple actually. Good luck.
This is pretty much exactly what I was wanting to hear. Would i just sort of free hand screed it while trying to keep it right at my level mark?