You've probably had better ideas.
1. The weight increase is a killer, right there, and that doesn't include the added weight of the larger prop further forward.
The O-470 is a physically longer engine, too, as well as being heavier.
2. As mentioned, you'd need to come up with your own engine mount, FWF and engine installation kits, and probably a modified cowling, too.
3. One of the advantages of the RV-14 is its complete integration and relative ease of construction. You'd lose most of that and add a couple years to the project, ending up with an airplane with much lower value than you should.
4. I fly an O-470 and it's got approval for auto gas. I find that on trips, it's mostly illusory. While the advent of the Rotax-powered airplanes with their auto-fuel requirement is changing this situation, you can't really count on the availability of auto gas at just about every little airport you stumble into as you can with av-gas.
And after considering the fixed costs of owning an airplane, the small difference in fuel cost is honestly not something I notice. It's just lost in the overall cost.
Bottom line, I've got to recommend that you not do it.
Dave