How often and long do you ride 34:19?
I live in northern NM (similar terrain to the front range at 2000+ higher elevation) and generally ride 34:18 here, and I ride it often. Last year at OP I pushed 34:18 the whole race, it did begin to wear on me (it is a 24 hour race though), and made me think it might be too tall, but in the end all was good. If the weather had been better, it might've been easier, but then you have to go faster and you still get tired...
So, if you're able to do longish rides on the 19, and ride a few days back to back over the same terrain without feeling that it's ripping your legs off, then you should be able to ride it at OP without too much difficulty. otherwise, a 34:20 would probably be the most that you have to back off. This might be boring at times, but most of the course is tight and twisty, which makes it difficult to maintain momentum, so you sprint out of lots of turns (or at least accelerate out of them)