Aaron, do you already have plans and materials for your bridge? I ask because Plastruct has a very nice O scale truss bridge kit, #3002 that lists for $39.95--it's 30" in length, but Plastruct states in its listing that it can easily be widened and lengthened
https://plastruct.com/shop/tra...-kits/3002-kit-3002/
FWIW, I made this kit in O scale before Plastruct offered it in that scale. I purchased the HO scale kit, had the diagram/plans enlarged to O scale and purchased the equivalent materials. Turned out extremely well IMHO. I'm currently investigating how to lengthen the bridge since it was damaged at both ends during a move--if it needs repairs at the ends, why not investigate making it longer?