PaulS, he said it had a carburetor back in the first post, so he doesn't have access to a data bus to monitor things. This is an old school bike that he wants to fancy up. EFI was an option from around 96 on, but not everyone got it. There were a lot of riders that felt uncomfortable with a machine they couldn't fix on the road. EFI is hard to fix at a freeway rest stop.
Hooch, this project is really cool. I have a Road King Classic and love it. I left carburetors behind because I was sick of having to adjust them when I changed altitude, but I totally understand your desire. Regarding the speedometer, I would use the GPS, more fun, more accurate and cool looking. For the fuel, put a voltage to it and measure the current from low to high levels. This will give you a feel for how much change there is. You can move the float by using a coat hanger wire through the fill cap. Failing that, drain the gas and check empty, then fill it back up to check full. Note though that the tank is non-linear. The position of the float doesn't truly correspond to the amount of gas left because of the shape. You can fill it, drain it into a bucket to see exactly how much it holds, then add back in increments to see what 1 gallon gives you, two gallons, etc. Then work up a table of values from there.
You can measure the voltage with a simple voltage divider. Don't need a shield for this. You want to divide the voltage down to the range an arduino pin can read and then just read it directly and multiply by the proper factor to get the voltage. RPM on a Harley is a bit weird, it gives two pulses for each revolution; the details escape me, but there are a number of sites out there that discuss this in detail. Look for web sites that discuss measuring RPM Harley. I found the details and how to overcome the problems on an old bike of mine that way.
For testing your various ideas, use the serial output. Plug your laptop into the mega(when you get it) and just output what you see in regular text. Things like, "Read 1.2 volts" or "got seven pulses" will give you what your need. Once you have it working, create whatever you want and display it to whatever cool device you use. See, this way you can change the code and reload and reread the data without doing anything special. Once it works like you want, then you can put it on a pretty display. Leave the debug code in though, you'll want that again later.
Think about an oil temperature gauge; you may want that on hot days. Heck, I would install a air pressure sensor in each tire and put the receiver in the faring to tell me when the tires went low. Never did that, and it's really pricey, but it sounds like fun.
One other thing, the pin sockets on an Arduino won't hold up under the vibration. So, just plugging in wires to figure out what to do and where to hook things is ok, but don't trust it long term. Solder this stuff on. I've tried little computers on a motorcycle a couple of times and that was my biggest hurdle. The darn stuff kept working itself loose. I have a cell phone charger on mine that gave me fits until I just gave up and soldered the darn thing in. Haven't had trouble since.
Keep the thread updated on your progress. Inquiring minds want to copy your ideas.