If it's your own creation, you should be able to adapt your model to fit the rack you have. One tricky problem I can see is if your construction is built as most studless models are nowadays and features uneven dimensions - which means a 7 rack is indeed easier than a 8.
You can of course build something around the old 1x4 rack place but it might get awkward depending how you want your model to work. Something like this might already work fine:
Also, I wouldn't discount the worm screw, as it can work as a rack as well:
Of course, this is just an example; with this solution you have a lot of freedom as to what you will connect at the ends of the axle, as well as to the length of it. Again, depending on your model, you'll surely be able to find something that suits your needs.