At 5w4d pregnant my hCG level was 8, 250. At 7w3d pregnant my hCG level was 36, 350. Saw heartbeat 7w3d at 158. Concern: levels rising quick enough?

Yes. Once one see the heartbeat, the levels become less and less important. There is wide variation in the "normal" numbers of hcg. Thus, now that you've seen a heartbeat i wouldn't worry for one second that the pregnancy is progressing appropriately. Good luck.
HCG. Hcg levels are quite variable and there is a wide range for weeks of gestation. A positive rate and a large jump in thehcg like you've experienced is a good sign, and everything is likely to be fine. Following up for regular visits and reaching the milestone of cardiac activity heard at 12 weeks usually indicates a greater than 80% chance of carrying to term. Good luck!