Sorry for the confusion. I have already made a custom template with the larger barcode. I just need some sort of workaround to set the new custom template as the default packing slip template so we can continue using the print labels w/ packing slip feature. I don't want to have to change my shippers workflow to involve printing the labels and packing slips separately if possible.
Totally with you there my friend! You can choose your default packing slip on a per store basis.
Starting at Settings (gear icon in the top right) > Selling Channels > Store Setup > Click the 3 dots under actions > Edit Store Details > Packing slips tab to choose that default. You can check out a screenshot here if you are a visual person like me 🙂