My suggestion is to work backwards.
First see if you can find the HMC6343 datasheet and register datasheet. Sometimes those 2 sheets are combined and sometimes they are separate.
Read the datasheet to see if the specs of the device falls within the range of your project parameters.
Look to see if you can find several libraries. If you can cool.
If you do not find libraries can you write a library?
Evaluate several devices. Pick a device and do the thing.
Also, you should consider which communication protocol you are going to use. I2C, SPI, UART, CAN or Joe's as part of your selection process.
Regarding a strong electromagnet and a Magnetometer, be aware that exceeding the devices rated field strength can result in the device being rendered useless.
Something you may find useful, to convert uT to mG multiply the uT by 10 for mG.
You should calibrate the device with the strong magnetic field off.
Know that if the calibrated isocenter is moved the calibration done will be 'useless'. If you are going to move the device calibrated isocenter you may want to consider a self calibrating scheme.
By using 2 devices with each plane mounted 180 degrees out phase and summing the measurements of each plane, you can have a self calibrating magnetometer. You would want to consider this in your initial device specifications.
Things like how much will 2 devices cost may be an issue. Also, can both devices be setup to run on the same buss, will, if using 2 devices, be an issue, will the I2C buss be fast enough to give a desired output, along with other device considerations you may have.