The Flint motors would be pretty difficult to be cast & machined the same day due to the trucking required between Saginaw & Flint.
That captive fleet of trucks ran 24/7 from the Saginaw Foundry to Flint V-8, delivering about 55,000 raw castings per day (blocks, heads, water pumps, flywheels, crankshafts, camshafts, intake manifolds, thermostat housings, etc.). The few blocks I've seen personally that were cast and machined/assembled the same day were cast during the first two hours on the day shift at Saginaw, and machined/assembled on the second shift at Flint V-8. Cast one day and assembled the next day was quite common in the mid-60's when everything was running all-out at Flint V-8; 300 engines per hour with Machining running 3 shifts and assembly (170 per hour on Line #1 and 130 per hour on Line #2) running two 9-hour shifts, producing 5500 engines per day (one every 12 seconds).