don't want to hijack a thread, but .....i'm about to pull a good running 273 and 904 from my dart. you can come take a ride and see for yourself. send me a pm if interested.
altitude may be your only problem. i would not worry or mess with this car til i got it home. years ago i was out in denver in a '71 duster with a good running 318. at that altitude it was an absolute dog. it took a mile to get up to 60 mph.