Yeah, what he said. Here's some more detail on Series Production racing, for example the Hardie Ferodo 500.
The 351 Windsor GTHO debuted on the race track at the 1969 Sandown Three Hour. Moffat and French won and there is not much mention of engine problems. However Street Machine mentions that engineer Ian Stockings had to do some work prior to Bathurst that year to get things right. As we know, the GTHO probably should have won Bathurst if not for the tyre issue, but the engines seemed fine. Several privateer GTHOs took on the HDT Monaros at the Surfers 12-Hour race and ran reliably, almost beating Colin Bond. Moffat and Gibson ran them in sprint races in the early part of 1970 often winning and without many engine problems it would seem.
However the Monaro was giving them a run for their money and the new Cleveland was in the wings with reports of better power and more revs. Whereas the Windsor with its small-breathing heads and hydraulic lifters would start to run out of breath at 5500rpm, Al Turner was telling everybody the Cleveland would go to 7-7500.
So before the 1970 Sandown 250, Ford introduced the Phase II with the new motor. Sandown almost turned into a disaster. Clevelands blew left, right and centre and they had to set a rev limit of around 6000, just to make the engines last through the race. Moffat won but there were plenty of others that DNF. Ian Stockings went back to the engine room at Ford to sort out the detonation problems and they managed to get things together for Bathurst. The works GTHOs of Moffat and McPhee managed to out last the opposition, but there were plenty more busted Clevelands and the works cars were in conservation mode for the last part of the race.
The Phase IIs ran for the first part of 1971 and ran reliably apart form oil surge related blow-ups, especially at Warwick Farm where the cars never seemed to be in a straightline for very long.
For the 1971 Sandown 250, the Phase III was introduced with revised oil galleries and bearings amongst other things. However, neither of the works GTHOs finished, and many other Ford teams had engine dramas. The Ford team went back to the engine room again and found assembly related problems. Al Turner's replacement, the late Howard Marsden offered Ford runners special QC engines for a mere $250 each which had been carefully assembled by the works team. Hey presto! The GTHOs were super reliable at Bathurst that year and it would seem from then on there were only occasional problems.
According to John Wynne, who was foreman of Ford Special Vehicles at the time: ?We had all sorts of problems with the crankshaft in the Cleveland. We didn?t know anything about them when they first arrived, and I don?t think the Americans knew much either and they were the ones who developed the engine in the first place. We found that under high stress there were fine grit-like particles that would tear the bearings apart, the engine would then lock up and it would spit a rod out through the side of the block. Once we found the cause we started linishing the cranks until the surface was perfect, they were then fine and we started to get good horsepower from them.?
And after that tyres became the GTHOs Achilles heel, until the new Globe wheels were introduced.
So yes, the Windsor was probably more reliable.