500 cubes is borderline between SRs and 440-1 heads, or B1s. Each will do a good job for a certain application. If you get a copy of AndyFs book How to Build max-performance mopar big-blocks, he has a chart of the three most common port sizes and where the torque peak comes on for each at different motor size.
I would be shopping for Eddys up to 675 hp, Indy heads or Eddy Victors for 700 to 900 hp, or if I am going for 900 hp and above, B1s.