if I'm not mistaken, a dynapac is a different style of dyno that what most are used to. Two packs are bolted to each side of the driven wheels and measure hp/tq directly from the hubs. The main down side to this is special adapters are needed for each vehicle.
A chassis dyno is one that a vehicle is strapped onto. Hp/tq is measure through the rear wheels. There are different brands, mustang dyno, dynojet, etc. This is often the most preferred method of testing vehicles. Chassis dynos come in both in ground, and above ground.
Mustang dyno's and dynojet's measure differently. check out their websites for more info on that. If you're going to be tuning your car, do it on the same dyno with the same operator that way you'll get similar results.