You can do exactly what you want. The Montana can make a great USB puck for your PC, with excellent sensitivity and accuracy.
Garmin Chartplotters and model 478 are totally different devices from your Montana, so their advice is not really relevant to you.
I have the Garmin Oregon, which is very similar to your Montana. The issue is that most programs (Polar, OpenCPN, and other computer software) expect to see NMEA, which is a serial protocol (COM port), but the Oregon/Montana output on USB exclusively. The best way to overcome this is with PC software that runs in the background and takes in the USB feed and redirects it to a virtual serial port. The best free software for this is Garmin's Spanner, which you install on your PC, and then put your Montana into "Spanner" interface mode. The Spanner software creates a virtual COM port to output the data, and you set Polar up to receive the data on that COM port.
Only problem is that Spanner is not formally supported for Windows7 or later. You might get it to install on 32-bit versions of Windows7, but I have not tried it on anything other than XP. For Windows7 and later, Garmin recommends GPSgate, which has a free version that's somewhat crippled, and requires payment for full functionality. It's been so long since I tried it that I don't remember how it works - I used Spanner under XP instead.
I'd recommend trying to get Spanner to install. If your computer runs XP, you'll be in great shape. If you can't get Spanner to install, try GPSgate and let the rest of us know how it works.
Spanner: Garmin: Spanner Updates & Downloads
If you can't get either of these to work, you can find dedicated GPS pucks with their own software that will do this independently of your Montana. The GlobalSat BU-353 is very popular, and can be found for about $20 if you shop around. I'd pay money for an extra GPS puck before paying money for software that might break with the next Windows release.