All dyno results are dependant upon atmospheric conditions, Correction factors are used to standardise the results at a set air temperature, altitude etc.
This is my local engine builder. He's not a Mopar guy as such, but that 273 had some good tricks in it. He also did a 500 horse 349 (318-based) stroker for a circuit car, and doubtless a few more.
I had him balance my 318/390 rotating assembly, and it will go on his dyno when I get around to firing it up.
