The negative battery cable grounds the engine block; you should also have a braided ground strap from the engine to the firewall. This should be good and strong because it's the ground for all lighting and instrumentation in the cab and the front clip. I believe the frame grounds with a braided ground strap to the firewall (the frame is not grounded to the engine through the engine mounts as is commonly thought, because the engine is mounted in thick rubber). My '79 doesn't seem to have a ground strap from the bed to the frame (that I can find at least); if that's the case then I'd imagine it's grounding through the bed bolts. It certainly doesn't hurt to add another ground strap there though.
Last but not least, if your truck came with a radio it will have a ground strap for the hood. This provides a larger ground plane for the antenna. It also serves as a ground strap if your truck came with an under-hood light.