what would be a good gear setup to run behind a built 289 (hopefully about 300-350hp) with a 5 speed OD transmission?

what is the stock rearend gear?

 

What are you trying to do? Good gas mileage or high performance? Ever gear set is a compromise between the 2. The best all around ratio however for the 5 speed is around the 3.40-3.55 range

 

I agree, I'm very happy with my T5 and 3.55 gears.

 

I agree with all those that said around the 3.50:1 range depending on desired application with one exception, which T5 are you running? The reason I ask is that 1st gear is different depending on the type of T5. I have a Z spec T5 and the first gear is 2.95 so I can run lower gears in the 3.50:1 to 4.11:1 rage with better results and a still usable first gear. The majority of the T5s in 5.0 cars were 3.35 so at that point you need to do as other suggested and stay close to 3.50:1

 

I am running 3.55 with my T5 and 289. So far I am happy with it. I don't have a ton of miles, but it feels pretty good.

 

Daze, aren't there a number of different 5th gear ratios for T5's as well?
Just a thought...to make sure the car is not running in that 1900 -2100 rpm range in 5th at highway speed so as to avoid droning exhaust.

 

Not sure about the T5 but in the TKO tranny you have the option of the road race 5th of .85 or street .69.

 

My T5 has the 3.35 first gear, and the .68 OD. I think it matches quite nicely with my 3.73 rear end, though a 3.50 would have been ok. All that is behind a 408W making 400 hp, 500lbs of torque at the rear wheels. It really jumps up and runs, but its not too bad on the highway. I just drove it home to Texas from California.

 

My T5 is a "200" code, with 2.95:1 first and 0.63:1 OD. I have 3.25:1 gears in the 9" but will likely switch them out to the 3.75± range.

Here's a chart to look up your gears:

http://www.allfordmustangs.com/Detailed/349.shtml

 

Are you back home for the summer David? Are you gonna go back to Kali this fall?