Dunkirk, film by Christopher Nolan

"M26 tank had a X24 (designation not listed) gas engine made by Chrysler...HP was de rated to 700 hp, and then de rated again to 570 hp to prolong trans life!"
Okay so this was NOT the A57 engine. It was indeed something else. This had 24 cylinders vs. the A57's 30 cylinders and 570-700 hp vs 370 hp so we are indeed dealing with something else entirely... It might be this one: A-65 V-12 (link below), experimental, high horse power and developed on Chrysler dime...

"...'Chrysler on their own dime came up with a water cooled, V12, tank motor...These 1568 cubic motors started out at 650 horsepower at 2600 RPM and 1485 pounds of torque at 1600 RPM on the test stand. They came in around 3840 pounds, but there was a proposed all aluminum version that would have dropped nearly 1000 pounds. Designing and producing the prototypes, cost a grand total of 358,000 bucks, that’s over 5 million in today’s dollars...They used an M4A4 as a test vehicle, and had to stretch it another 9 and 1/2 inches to fit the new motor. Installed and ready to roll the thing came in at 69,170 ponds, and a stock M4A4 came in at 69,640 pounds! Installed, the early versions had 549 horsepower, but they upped the compression ratio and got it to 580..."

It would still be a heavy bugger when you get the new turret and 90mm gun on it... I need to dig some more on this motor but no time right now...

Gramps was indeed in the 2nd Armor Division which makes sense...

#68 The Chrysler Engine that could have been: The A-65 V12, Chrysler’s home designed tank motor. | The Sherman Tank Site

Chrysler builds Sherman, Pershing, and M3 tanks for the Arsenal of Democracy