My toilet is running and gurgling and water level is lower than it should be. The toilet is approximately 40 years old. Do you think the flapper needs to be replaced? It seems tight but... any help would be greatly appreciated.

Sounds like you have more then one problem.
First Off I have to know exactly what's running. Is it the ballcock running over into the overflow tube or do you hear the sound of water leaking past the flapper?
Take your hand and run your finger around the flat part of the flapper. If you feel nicks, rough spots or wavy edges replace the flapper.
Your gurgle is caused by a blocked vent. Them sound you hear if the fixtures attempting to vent through a trap in the toilet.
The solution would be to snake from the lavatory roof vent. Put out enough snake to reach the base and 18 feet more. Good luck, Tom