Hi Milt.
Although Im not rated on type. I have witnessed a number of times, full power run ups of a Mustang whilst fully tied down as you explained above.
The Merlin changes tune somewhat when the power is above 2300 RPM!!
Last time I recall running at 3000RPM and 61 inches was after a prop overhaul, and to ensure the governor was governing at rated 3000RPM when full take off power was required! Sounded so, so goood!
Cheers.
K-Ex