Caley, First of all you cannot use the two different packs together. You must use battery packs with the same current ratings in order to avoid a potentially dangerous overdischarge. So I would recommend the 5000 size packs and then add 2-4 extra cells for the required output power.
Next is to determine how much power you really need for this plane. The real J 3 Cub flies with the equivelant of about 50 watts /Lb. using a Continental 65 HP engine. So now we can determine that a motor with a rating of around 1600-1800 watts would be more than suffecient for this plane given the power loss due to the efficiency of the motor. Some motors are more efficient than others. Outrunners are not as efficient as the inrunner types but you will need a gearbox for an inrunner to be able to swing the large prop needed for that plane. Which brings us to the outrunners. Since no gearbox is needed, you can decide what size and especially how much money you have to spend as these motors now come from manufacturers from all over the world, especially China. These motor sare inexpensive when compared to the Axis, Hackers, Plettenbergs and are not as accurately wound, neither is the materials used as good. There are some exceptions: Hyperion Motors from Japan are very well made, have good efficiency and are not as expensive as the motors made in Germany, Cech Republic.
Another brand now coming on is the Scorpion Motor. Although this motor is made in China, its designer is German and strives to obtain and use the very best materials including high temp wire and magnets.
For this plane, one of the larger Axis in the 50** series, a Hacker A60 series, Hyperion in the 50 series will work. I used Ecalc to estimate a motor for this plane and came up with a Hacker A60-12M on 8-10 cells. Just remember that this plane flies on the wing and doesn't need the power of a 3D or warbird. Around 70-85 watts /Lb will be sufficient. I would have to check back with my Ecalc records to see exactly what I came up with but I do believe an 18" prop was used.
The Axis are not the best in efficiency they lag behind the Hackers, Pletts and Hyperions. Quite a few people are using the less expensive motors from Chinese manufacturers but, you get what you pay for. Quality control is not the best, especially when it comes to windings as two motors of the same size and specs will measure different KV and winding resistance. Still a lot of people use these motors with some satisfaction they spent less money and get adequate performance.
http://www.allerc.com/
http://modelmotors.cz/index.php?id=en&nc=domu
www.Aero-Model Inc.com (Hacker motors)
Some other brands such as Turnigy are popular as they are not as expensive as those mentioned above. Plettenbergs are by far the most expensive but they are the best built and most efficient of the outrunner types.
http://www.icare-rc.com/. You will spend a lot more money for these.
Don't forget to use the appropriate size speed control and either seperate Rxcr pack or switch mode BEC unit for your receiver and servos.
Kepp us posted with your progress.
cheers