@jake38311
To answer your specific question. No I wasn't in Engineering at the Harvey plant or the Mattson, Il plant when that engine change was made. But I do know the details behind that engine change (I was a Product Sales Manger/District Sales Manger at that time. To begin the AC lift truck division in Harvey moved to a new facility in 1969. The largest customer for the Aliis-Chalmers 4G153 & D153, plsu the G262/D230 was the US military. The engines were used to power generator sets. The gen sets were widely used during the Vietnam Conflict. As the Conflict began to wind down so did the order for gen sets. In 1975 the US bailed out of the Conflict With any product you have to maintain a given production volume to keep cost under control. The cost to buy the engines for lift truck use went up and up and up. At the same time at the Mattson plant there was a corporate edict to reduce costs. It was called CRAM (Cost Reduction At Mattson). See David C Scott waned/needed tons of cash to fund his pet and ill fated coal gasification project. The lift ruck division choose to go to the flat head engine while the US competitors were switching over to OHV engines & Japanese models were hitting the beach in California.
PS: The Vietnam Conflict was declared a War until well after the US pulled out.

Around 1980 A-C returned to the OHV AC branded 4 & 6 cylinder engines (gas/LP). Late '81 I was asked to leave and in a few years the plant closed - it died the death of a rag doll as an old boss of mine, the late JR Barber, would say.