Thank you for your questions and comments. I'll do my best to answer what I can.
Let me start by stating that this car never raced AMLS and was never fitted with the P60 B40 engine.
The car was specifically built to run in The "Rolex Grand Am" racing series. Classified as a GT. Not an AMLS GTR which ran the P60 B 40 that you mentioned. As you know all of the engines were returned to the factory and are not available to the public. This car has "always" run the S62 from new. Which makes it much more attractive to me as the car owner as parts are readily available at the moment.
This car was an effort of 6 cars from PTG and BMW "North America". 4 of which ran S54's and 2 S62 V8 cars. The V8's were built specifically to beat Porsche in Grand Am GT class. The car you see here is one. The other V8 car is somewhere in Australia. I'm told it may have been destroyed in a wreck at Bathurst. Not 100% sure.
Somebody should start looking for it.
As for the horse power. I am simply a repeater of data that was provided to me by PTG engine builder and tuner after dyno runs. As for how they get that type of performance. They will not share their secrets with me. Note that the engine is currently running "unrestricted" a luxury not allowed race teams under various sanctioning body's. Also note that this car ran 2 different engine set ups while professionally competing. A mild and a wild engine. This being the later. Estimated engine life I'm told is 30-40 hours before tear down. The block can be torn down twice from new. Then its scrap. Typically the block will fail before the 30 hours mark on the second rebuild.
I am also told the drivetrain % loss may vary as the result of the Hewland 6spd straight cut gear transmission and the selected gear set. The car came with 25 different sets of gears.
As for the suspension and rear sub-frame. There is a difference between the GT and the GTR. The GTR being more developed. I am told that they share some similar components such as magnesium rear control arms etc... As for suspension. I'm told that it was specifically designed for this car. I thankfully have a large box of spares in case of an off track excursion. Beyond that I'm unable to answer your question without calling Tom Milner.
I am not an engineer or technical guy. I am however an enthusiastic car guy and driver. I truly like this car and find its history interesting. What I have attempted to do is refresh the car as though it was going back on track in. 2001. But. Taking the liberty of max available horse power. Why not enjoy the ride. As this car is currently a Vintage Racer it is not bound by any restrictions. Hopefully I have answered your questions