It typically takes an entire day to get an engine bolted up to the dyno, run and then stripped back off so $300 is very cheap. I'd think that a shop would need to charge at least $500 or more or else they'll go broke in the long run. A one man shop isn't getting any work out the door when they are running the dyno so it gets expensive. Perhaps a bigger shop has a junior mechanic who can bolt the engine in place and pull it back off so that cuts down on the cost.

If the shop knows how to run the dyno then they'll get all of information for you but you should talk to them about headers, ignition setup, type of fuel, how the dyno hooks up to the engine, etc. Lots of little details to sort out. If they haven't run a Mopar before then they might need to make some adapters or rig up some stuff.