I would take what JL specs say with a grain of salt. I would only look at that as bare minimum, not ideal. You could technically do a 12" ported in your application but I would try to aim closer to 2.0 cu ft if possible. Using the dimensions 12.5 x 12.5 x 40, you can achieve a pretty good response from that 12w6v3. You can use a 4" flared aeroport on the side to accommodate.. Another option is you can use 2 12" passive radiators on the side. This will use up the least amount of internal space and will still be able to achieve your 34 hz tune that you're looking for. I personally am a fan of PR boxes when space is so limited.
Creative Sound - Product Details this is what I'm talking about. Someone on this forum recommended these to me before and I absolutely loved them while I had them. It does cost a bit extra to get them into your plans but it's well worth it for space saving purposes while still being able to get the response you're looking for.