Rear end advice for racing with a 4 speed
In the photo of the housing getting narrowed up. That's a Dana 60 HD housing. Those are boat anchors with and not something I would ever use under a car.
Getting a housing from 60's or 70's Ford F250 is a great place to find one, but you have to be careful in the mid 70's up because you could end up with a Dana 61.
The later they get past the 70's the heavier the housings get and worse yet the axle tubes start getting really thick. Some are close to 3/8" wall.
Tom