You can add slightly more bags to the bed if you lay two layers flat...and...then stand the rest of the bags on end, (on top of the 2 flat layers).
I had a welder modify my skid sprayer to make it four inches more narrow. And then I had room enough to carry the spreader in the back left corner of the bed. A smaller spreader may be a help in such a situation. Maybe a 50 pound capacity spreader suits the situation. No point in carrying fert if you can't get the spreader aboard.