With say, p255 street slicks and a 3.89 gear posi, How much horsepower and torque can a ford 8" rear end handle before it snaps?

 

Many factor in finding the limit of a rear end.

As you mentioned, torque and street slicks. Definate recipe for disaster if you launch from a high rpm.

In your case I'd say low 13's is approaching danger zone.

Weight is another big factor and that is on your side.

 

the housing should be ok...but eventually u might wanna get a better drive shaft and axles