In all cases when attempting to create and print such a project, it's best to just use cheaper letter paper, cut to size if needed, until you get everything sorted out so things print as you want.
Even if you use and set a custom page size, you can still just print to normal and cheaper letter stock.
EDIT:
I was going to suggest using Avery's free Design&Print.app but it keeps crashing on my iMac and I have to re-install it to get it to work.
Their iOS Avery Design&Print app seems to work better but I can't find any setting for Avery 5 x 7 fold-over stock, and gave up looking.
The OS X Design&Print version has a custom size option and provides the option for inside/outside printing so one doesn't have to rotate any text or images for a single print job.
Otherwise you may want to use two separate templates, one for front outer and the other for the inside on "2nd" fold-over side.
Yes, card printing can be a real PITA to get everything correct.
- Patrick
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