First off, I agree wholeheartedly with amberwolf ohbse and spinningmagnets. First I'll share a story from my experience, and then I'll give you my recommendations on making your dream possible (albeit with a couple major changes)
One time I was on a commercially produced gas moped, (~150lbs, 65cc), which had massive suspension etc, easily over 4 times as sturdily built as a bike. I was going down hill, and when I got past 50mph - it was insane. Insane. At 65mph the steering wobbled so much I was lucky to make it out alive. A regular pedal bike at 50mph is a good way to end up in the ER if you're lucky, or dead if you're not. Sure, someone, somewhere, might get irked, and post to chew me out about how great a 50mph ebike is, but come on. If you're running that kind of rig, you have a heavy duty frame (probably custom), with massive suspension, fatty tires, brakes etc. It's not really a bike at that point, it's a motorcycle, although maybe a lightweight one, and legally still an electric bike because you "can pedal it".
Either just realize you are talking about an electric motorcycle, which is totally fine and doable... (and ditch the idea of a geared motor - you obviously need to do a LOT more reading and research if you thought that was gonna work)...there are even electric hub motor motorcycle wheels commercially available...
...or downgrade your need for speed. An ebike is really best in the 17-30mph range, unless using beefier tires/components. Even with beefier stuff, 45 is really the upper limit IMO, and even that isn't entirely safe (it's only really safe for straight stretches with no risk of sudden cross traffic/stops).
BUT if I was going to attempt this (50mph+ ebike) - here are my tips:
1. If you are set on a 2 wheel upright bike, go with 2wd (hub motor in front as well as another motor powering rear wheel) Why? Not just traction, although that is very good for safety at high speed as well (notice race cars have wide tires), but also a major benefit is the spinning weight in the front wheel will stabilize your steering at high speed. You do not want wobbles after you sink $1500 into your beast machine and realize it's an overpowered moped which you can only ride safely up to 40mph. Avoid the pain of overbuilding, only to realize it was wasted. This may also making steering more difficult, because there is some resistance, but at 50mph, you need all the stabilizing you can get up there. Non spinning weight would stabilize without causing the steering difficulty, so maybe load the batteries on the front fork somehow? (I've never seen it done, but it seems like a decent idea to me, as long as it's not in the way of steering)
2. Personally I would want a low-profile recumbent with full fairing/shell. Human powered vehicle land speed records are always set on shelled recumbents, for a reason.
Aerodynamics. Much easier to reach high speeds. The current record: 83.33MPH Yes you read that right. MILES per hour, not kilometers. Just pedalling. Can you freaking imagine? At that point, they are basically lying down flat.
At 25mph wind resistance becomes significant. At 50mph you are spending more energy pushing air out of the way than you are hauling weight across the ground.
This is a really helpful page about wind resistance speed increases:
http://phors.locost7.info/phors06.htm
The table at the bottom shows that at 30mph you are using 1.16 horsepower. At 55, you are using 7.14 horsepower. Think about that. From 30 to 55 isn't even doubling speed, but the jump in horsepower, from 1.16 to 7.14 means it's using more than 6 times as much energy. It's really ludicrous. Hence getting a more aerodynamic setup. That's how folks can do 83mph on human power (less than 1hp, more like a bit over 1/2 a horsepower). Upright bikes are air hogs.
Another thing that's great about low -to-the-ground recumbents, is they FEEL fast. It's like a go-kart. 30mph feels as crazy as you want to go - everything is wizzing past and you lose all desire to go faster. Also the lower center of gravity helps you feel secure, so if you did push the speed, you don't feel as exposed as on an upright.
3. There are way better motors out there, if you are thinking of a geared motor, you need to read a lot more. DD for speed all the way. I'd suggest crystalyte, followed by 9c. Do your research. Also, mid-drive could work, but less simple to install, and usually chain noise is audible.
For ebikes, I recommend sane speeds around town - up to 25, with capability of 30-35mph bursts as needed. If you want more than that, you might consider just getting a regular motorcycle. If you hate the exhaust, convert it to alcohol.
Also, one last thing - I make sure you run your build by folks on the forum before you run out and get parts. I don't want to be responsible for encouraging you to do something crazy. Read up a lot before you build something that pushes a device designed to go 17-30mph (bicycle) to 50mph. They aren't designed to be powered, let alone that much power. Imagine the front fork ripping into shreds from metal fatigue as you hit a dip on a downhill at 60+mph. It can happen. Hairline fractures can be invisible on the surface, and you could be dead in seconds.