A slow cranking motor will probably not be resolved by adding a Ford-type starter RELAY.It is normally used when you have is a solenoid "click" but no crank problem after a hot soak. Sounds more like you have a weak battery or poor connections between battery and starter or a poor ground from battery to block. Check all connections, have battery load-tested,and make sure you remove paint from the starter mounting ears so starter has a good ground. Another possibility would be that the engine got tight when it got hot, putting more load on the starter than it could handle.
Does your starter have a solenoid attatched like a typical GM car? THe top mounts I remember don't. If yours doesn't, then the addition of Ford relay would not apply in any case.