Adding PI heads onto a non-PI motor will increase the CR by a full point. This is not the modification to do if considering adding boost.
Further. Knowing what I know now I would never perform a head swap with the motor still inside the car. Access is a PIA. IMO the chances of success are higher with the motor on the engine stand and access is soooo much easier. Frankly it's not that much work to pull the motor in a Mustang anyway. Tons of people have done the job in their driveway with nothing more than hand tools and a rented engine crane.
IMO the whole motor swap makes more sense because:
- The cost of the PI heads are often greater than the cost of a whole salvage yard motor. Many people will buy a salvage yard PI motor just to get the heads.
- The labor and cost of parts (head changing kit) need to be factored in. The head swap itself is not a trivial job.
- The whole motor swap is straight forward. Mostly swapping bolt on parts/wiring from one motor to the other.
- Possible to get a donor motor with much lower mileage than original.
- Much easier to perform some preventive maintenance while on the engine stand. Think seals, gaskets, timing chain (anytime motor has 150K+ miles).
When swapping in a PI motor into a non-PI application a tune is needed to get the full benefit of the PI motor. Note, the PI motor will run and will give some additional power with the stock tune.