I agree with going with a 2004R, unless you're making ridiculous amounts of power, then a 4L80E would be my choice. The only way to really drop your RPMs will be a gear change or OD transmission. The best you'll get with a set of taller tires is probably a few hundred RPM lower than what you are, which likely won't offset the cost of just changing tires. Also, keep in mind how your motor was built. If the cam's powerband doesn't start until 2,000 or higher, you don't want to drop too far below that, otherwise you won't get decent vacuum to the carb and it will likely want to run rich.