Not sure about what size carb your planning on but, I think the extra distance might not be a huge problem. I would be careful about having more weight so far out on the manifold though, more stress on the bolts which hold the flanges on the ports. Do opposed's have a brace in the middle of the manifold somewhere? Not sure. But if your carb is too small, the engine would still run, just high rpm power would be restricted somewhat. Low end would be better, probably a more responsive throttle. You might be able to go smaller than you think before you notice anything.
If you did have trouble with running the longer manifold, you could try heating it slightly somehow, maybe with some sort of pipe which directs some heat from the exhaust. Just to warm it somewhat, doesn't need to be super hot. But I don't think you will notice too much difference. The extension piece being down draft should be a help.
(Not to knock what you said richie, as it is a long distance from the carb to the port, just adding my ideas)