I have a detached garage (20 x 32) that was built in 1929. I would like to replace the joist so I can have storage above as well as door openers for both cars. The doors I have are Barn Door sliders. I plan to replace one with a standard style and leave the slider on the other side. The existing joist are 2x4's. Approximately 30% of the joist are not single run. The rest are bowing a bit. The previous owners used it for storage above and has some 1x6's attached vertically from the raters. What would be the best way to approach this?