In every single case on all the GTO's I've owned and helped out with, slow cranking or no cranking when hot was due to excessive resistance in the purple S wire that runs from the back of the ign. switch to the starter solenoid. You can easily verify this by hooking up a remote starter switch and seeing if it turns over easily when the engine is hot with the remote button. If so, it can't be the solenoid or starter. It's the wire.
People don't want to hear this for whatever reason, and like to install mini starters, Ford remote solenoids, oversize cables, etc. But the actual issue is a bad 60 year old wire that can't handle the load due to degradation.
I can drive mine all day long in 100 degree heat, shut down and let it heat soak, and it will start right up. With a stock 40 year old Delco starter, no heat shield, and repro AMES spring ring battery cables. And a new purple S wire. The factory stuff works just fine with no embellishments needed as long as it meets factory specs.
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Jeff 
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