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It sounds to me, based upon your statement that when hot the engine is reluctant to turn over and makes a single or slow "rrr" sound that one of two things is going on...

1) you can discount fuel as this has nothing to do with the hot engine turning over. The fuel system is separate and would not balk the engine up at all.

2) A cast iron block is thermo-filic or absorbes heat. That said, a new or tight engine that has close tollerances will expand when hot and tighten the clearances. Thus the moving mechanism (Crank, valve train and cylinders) will offer greater resistance to the starter when hot and expanded.

3) The starter will then have a harder time turning over the tighter engine when hot. A borderline maximum advanced timing will just make this worse. This is quite common with higher compression engines that are new and not broken in. Is this your case?

So the fix is....

1) Make sure your power to the starter is up to snuff. The windings can prematurely age with heat. Perhaps put a heat shield on the starter.

2) Perhaps slightly back off on your ignition advance towards retarding the ignition. Reason I say this is that the ignition is probably the culprit being just slighly over advanced. This would cause hard starting when HOT. The FE is a counter rotating engine (anti-clockwise). You should scribe a mark on the distributor body and then carry this line onto your intake manifold. This will be a reference line for your current ingnition setting. You can then loosen the bolt for your distributor and turn the distributor CLOCKWISE (right) (to retard) so that the amount of turn is just one line width or about 1 or 2 degrees. This may solve the problem. Make sure to then tighten the distributor lock bolt. You can always go back to the old setting by aligning the marks. This should do it. This is a trial and error type of adjustment. Note: moving the distributor counterclockwise would ADVANCE the timing.

FWIW, I sometime have this same problem in really hot days in the summer. However, this does not happen to me in the cold of winter.

Hope this helps?