As matter of fact, mine is a 9/73. Vermont titled it a 73 and so it has always been. I have my manual, but it’s not handy to check. If I remember correctly, I had the 155 sticker on the head, but it’s been many years since I pulled that engine. I also think the oil filter was on passenger side, but there are to many alcohol soaked cobwebs to be sure.So you have a plastic not metal front grill emblem and front heater with the vent at the top? Heater and emblem were there just changed for the 74 model. Also for 74 the F155 changed to improved oiling system. Curious of your A pillar stamped build date. 9/73 was when the 74 model year started. States titling them by build date verses what a manufacturer say what model year it is was common. By this time the owner's manual have the model year on them. Have a original owner's manual for my 73.
I thought 74s had ambulance doors? I have a lift gate like the OP.