These 6 engines are the small block Jimmy family. Theoretically, any of the above heads could be installed on any of the. However, the 4. 77 head would be limited in performance by the. The 4. 17 and 9. By noting the photo, it is. H. head would make less compression with the 3. The 2. 19. 48. 19 head. None of. the stock GMC pistons is recommended for a performance engine, due to. Auto Parts and Truck Part for Car and Automotive parts.