A series race spec. I think the Ford English axles are ok up to around ~200bhp (yes I know its torque that matters, but bhp is generally easier to estimate.) A mate of mine ran same axle type in his heavily modded Escort racecar which ran a 200bhp Pinto engine and no diff issues.

I used 4.44:1 diff ratio and I think he ran 3.9:1. Wouldn't want to run the low ratio (ie numerically high) diff with a large/torquey engine as the axles/half shafts twist and break due to the torque multiplication.