As Old Sub pointed out....you didn't offer much information. If you're trying to determine you ground speed/rpm you might use the following formula to give you some idea of the benefits of a 3.42 R&P.
http://csgnetwork.com/multirpmcalc.htmlKeep in mind that the straight 6 Chevy engine is going to be happiest from about 1600-2400 rpms, although it can be run up another 500 rpms. These numbers put you into the torque band where the low-end torque will work in your favor. Plug in your rear tire height, the R&P ratio (3.42), 1:1 which will be your 3rd gear ratio and you can vary the speeds to see where you will be in relationship to the torque band.
I've found that my engine runs most efficiently towards the top of the torque band......1800-2300 rpms.
If you're asking about swapping from a closed driveshaft system to an open differential, you're going to be faced with a potential transmission change, a new driveshaft, and modifying the differential mounting system to bolt your Blazer diff to the stock rear suspension. You'll more than likely need to upgrade your master cylinder to a dual chamber and replace old brake components and adding modern shocks.
Challenging but not impossible if you have the time, experience, tools and cash to do the job!
Dave
