If the switch was made to the Pershing tank before D-Day, there would have been no American tanks at the landing. They would have been too big for the landing craft, and there would have been no way for them to wade ashore.
Add to that the fact that most American rail cars has a load limit of 40 tons. The Pershing weighed 43-46 tons. According to the book “Tanks Are Mighty Fine Things,” put out by Chrysler in 1946, the minimum load limit of the cars required to ship one Pershing was 118,000 pounds. That means a lot fewer tanks in theater.
There was also a shortage of flat cars of of any capacity. On Christmas day of 1944 the Chrysler plant had 75 tanks ready to ship and only 18 flat cars on hand. Even Shermans had to be shipped one to a car unless a 50 ft flat car turned up. If the US Army was building tanks as heavy as the King Tiger, it would have reduced the number of rail lines able to move the loads, further clogging the pipeline.
These logistical issues never seem to show up in armchair discussions of the M4 in WWII.
Good points. OTOH, they might not have needed all those tanks if they weren’t suffering such huge casualities in tank to tank combat. It wasn’t uncommon for a dozen Shermans to lose half of more of thier force in an encounter with two Tigers or Panthers before they even closed in enough to begin to fight back. Worse than the tanks were the crews. Tanks could be patched up or canibalized, but their veteran crews were gone. Since Lt. Coopers job was to clean out the splattered blood and guts of the crews and clean up the tanks for new rookie crews, he was obviously biased in the matter.
I suppose they might have used a mix of the two tanks, although that would creat extra logistical problems.
The M26 Pershing will indeed fit in an LST [not sure about a LCT] and the 70th Armor's DD Shermans hit Utah beach from an LST launch about half that of the Omaha landing, one reason the casualties at Utah were not as horrible as those at Omaha. And, of course, during the Inchon landings, the 70th's Pershings hit the beach again- their fourth wartime amphib landing. After 4 such jobs, they've gotten to be pretty good at it.