What kind of front brake is it? it sounds like it's just not compatible somehow. Alloy rims shouldn't be a problem, and are really a lighter rim. I've found good caliper road brake to be superior to cantilevers- I'm thinking your frame has no fork mounts for cantilever and you're going through the hole at the top of the wheel?
Just need a good brake, and it can be tricky on cruiser fames w/26" On the other hand, if you put a 27" wheel on, it not only rolls better, but a brake with a shorter reach will then fit- and generaly, the shorter reach brakes are the higher quality and better stop, but generally, they don't fit on a 26" wheel.
Heres the Campagnolo brake I put on my cruiser after I put a 27 X 1 1/4 wheel. It's a fantastic stop, and I got it on ebay cheap, but I don't recommend this model- a Veloce- because the reach is REALLY short, and I had to bore down the sides where the pads fit to get more extension. Also the center fitting is an oversized allen nut, which required more drilling on the fork hole. In short, there's no room for adjustment or error, but I'm happy.
On the other hand, a general road brake, particularly one that advertises itself as a long reach would be easier to install. I don't have any experience with BMX brakes, but that is what generally would fit a cruiser wheel and they're usually not very expensive- search "BMX bicycle brake" on ebay