Big Chief Heads
 

Has anyone used the Dart Big Chief Heads on a BBC with 4.94 inch bore spacing?

They will not fit without substantial and expensive modification.

You talking about 18* Big Chiefs? If so I don't think you would gain anything on a 4.9 vs a 4.84.
11* Maybe, but I wouldn't spend the money to try it. You's be better off with a true 4.9 bs head or even better a 5.0 head on a 4.9" motor.
One of our classes has an iron block rule which is why they don't use a 5.0 alumium block.