You will end up nose-heavy w/ a 20+ oz engine such as the Magunm/OS 70 4-stroke, not to mention ground clearance limit to drive you to a 12" prop that removes the advantage of 4-stroke on 3D planes. However, you can mount the battery at the tail. APC 13x4W is the best 3D prop for the 70 4-stroke on a place this size. It's also more scratch proof than a non-wide APC prop.

One advise for you man, ARF can be returned easily and re-purchased later at a more appropriate time. However, a wife is much more difficulty to re-purchase once gone. So weigh your priority carefully. Since you can't build 3 planes at the same time, I'd return 2 to sooth your wife's anger. Well, at least you can say you exchange 1 for the 4-stroke engine, and return another 1. Whatever you do, just remember that if you want to fly RC for the long haul, you need your wife's favor for the planes. Finding that balance, my friend, is the secret of a satisfactory RC.. errr I mean marriage life.