Is 74 pounds overweight for an 8-year-old girl?

Not if she is over 4'1".

That puts her in the high 80 percentile for weight. The important thing is to check her personal growth chart. If she's always been in or around the 80th percentile, then she is fine. If there has been a big jump (say, she was around 50th percent and then went up to 80th in a very short period of time) you might want to consult with your pediatrician. Another thing to consider is her own growth pattern. My daughter tends to pack a bit right around the middle before a growth spurt, and loses the extra fat after she gains height.