8 1/4 rebuild

Hi, I'm in the process of rebuilding an 8 1/4 to put into my valiant. I believe it came out of a 74 duster, but I'm not sure. This car is going to be a cruiser with an occasional lead foot. Right now it has 2.45 gears in it and drum brakes. I am going to be putting in a 360 magnum with either a 904 or 727 depending on what I can get when I'm ready for it. Goal for the motor would be around 400hp. I'm trying to keep it affordable and having trouble finding affordable new parts... With that I have a few questions...

1. I want to put posi in it, but I'm only finding the spicer trac lok at dr diff, or the yukon gear dura grip. The spicer says its good up to 350 hp and the dura grip is twice the price. Am I missing any other options?

2. Gear ratio, because it is a cruiser would I be doing a disservice to myself staying with 3.21 rear so I can keep my gas mileage up or should i go up to 3.55?

3. Brake options. I can't seem to find an affordable disc or drum brake option... has 10" now but they need to be replaced.

I have been searching for days for answers to these Qs and I haven't found the answers that I like. TIA