I am converting a 1987 blazer in to a baja ready vehicle. I am building up a 350 to drop in, but I am not sure what gear ratios i should use both in the transmission (5 speed manual) and the rear diff. I am getting ready to swap in a gm 12 bolt. let me know if you have any suggestions.

 

What are you hoping to get out of it? What speeds are you looking to hit? What is the power band of the motor?

 

Go with a NV3500 or NV4500 tranny, the 3500 will have better racing gears, however the 4500 is a stronger tranny, and has really lower gears, better for hauling, not hauling ***.

As for the rear axle, go with a GM10, or the preferred Corp14 bolt full floater. The 14 bolt full floater is the mother of all GM rear axles. Its found under 1 ton trucks and its bigger, stroanger and more common then the rest. Plus if you break an axle shaft the tire wont come off. With all the other GM rear axles the tire will come off. Google Full floating axles verse semi-floating axles.

As for axle gears, depends on tire size and like Swart asked, engine power range.

Is this going to be a 4x4?
If so you can also change the gear ratio in the t-case numerous ways. Everything from different gars, mating two t-cases together (doubling),adding a under or over drive spliter, or swapping the t-case out for an Atlas or other aftermarket t-case.

 

The engine has around 380hp and 350 pound feet of torque. It is a 2 wheel drive so i will not need a transfer case.
thanks for the advice.