I found that running multiple belts of supply with stackable conveyor poles solved my assembler woes.
You just run a stack of supply belts along the entire length of your row of assemblers, spaced 1.5 foundation squares away from the machine inputs. You place splitters directly onto the belts where needed. Splitters can overlap so you can have them all neat in a column if you wish. Beware that the long supply belts can not be placed again if accidently deleted them because of clipping through the splitter. To avoid this issue, alternate one empty stack space for each belt in your coveyor pole stqck. You might need to increase the foundation spacing to account for the change in height, however.