Personally,I think an engine dyno is the way to go.
I put a lot of research into it,and bought a Superflow engine dyno so I could do alot of testing and hopefully learn some things to help myself,and customers and friends out.
Some people will argue that chassis dynos are accurate,but I disagree.I won't eat up this whole post with why,so I will leave it at this;
An engine dyno that is properly calibrated and run by an operator with the correct procedures is the most accurate way to test an engine.
I hope this helps you in some way.
Keith