Some things:
When using ducts, make sure not to make them very long, because the duct itself puts a resistance on the airflow and your fan will loose function because of that.
However, my guess is that it won't be neccesasy to use ducts...
The coolling of a cab is based on the chimney effect. There should be openings in the bottom to let in air and near the top to let if flow out. So be sure that there's no blocking of that air flow.
Do you have your PC closed inside the cab ? May I ask why ? Remove the cover !
It's already in a casing(the cab !!) so why need another computer case ?
My Jamma cab had a horizontal slide-in board to mount the original game PCB's.
I have installed my PC components directly on that board, best cooling you van get

Like this:
(looks a little messy on this pic, but the DVD-drive is there only for installing, and there's some more loose parts still...