My other vehicle being a fire truck I can assure you that you will have no problem being heard. I am not familiar with the brand that you speak of, but am assuming that it is a little older. That alone will guarantee your audible success. You see, before the government got all concerned about workplace stuff, there was little regulatory oversight. Tinnitis (nasty long term ringing of the ears) changed this, as the older sirens cranked out some serious decible levels (probably somewhere in the 130Db) range. This level was necessary due to the older firetrucks being straight piped and very loud.
The place I worked before had a old Seagrave open cab that when fired up and driven through town, could be heard on the other side of town. Pretty cool!
The only caution that I offer is hard won. When driving the Big Red Truck at work, people get sudden stupidity when they here the horn and usually freeze for a period of time. Also, as fun as it may seem don't scare your loved ones as they walk in front of your vehicle as to the uninitiated, it may cause loss of bladder control!
Local police. Don't worry about them. They don't care unless you get stupid. Although MSP is rather annoying. I have land up in the Garrison area and have been nailed more times than I care to remember for speeding!