Hello Batwrangler,
You should have enough info now to verify what heritage your '73 has.
Your chrome "Powered By Ford" valve covers are more than likely chrome plated original blue covers. There were no standard or optional chrome valve covers offered by Ford for the '71-73s. The closest production cover would be the chrome "Powered By Ford" valve covers on the '69 Boss 302 and some early production '70 Boss cars. They had the oiling fingers since those were solid lifter engines. To my knowledge, Ford never released a service part number for these and was discontinued when the finned aluminum covers were released that we usually see on '70 302 and '71 Boss 351 engines. These are rare valve covers.
I won't get into the numerous part and engineering numbers on the pre '73 engines as you can about drown in a soup of numbers. As Don C mentioned, occasionally, Ford would use a casting number for other applications, which would drive number chasers crazy. I've witnessed some very heated discussions between some Boss 351 owners who claim to have found different ID numbers on their original engines.
Checking for cylinder head identification can be as much as a PIA as finding out what our Government is hiding in area 51. As Don noted, it required pulling the intake as the ID is under the intake port on the head. I bet the engineers still get a good chuckle over that.
If your engine is original to your Mach 1, then the heads would be marked D3ZE-AA. That head had the 4bl port size, smaller 2bl valves, and a 75.4cc combustion chamber. (CC number varies by information source).
The price of the illuminated I phone scopes has come down where they are now an affordable tool and can be used as Don suggested doing a main cap bolt count. Unless your vehicle had an engine change or some Ford repair work at some time, then you should get some good answers. At the end of 351C production, the D0AZ-6010-C (2 bolt block) became the service replacement block as inventory was depleted. You can verify your engine as OE by checking the left rear of the block right below where the cylinder head mates to the block. If original, there should be a partial VIN starting with "3F" and the six numbers from your consecutive serial number. A good light, mirror, or camera/phone, and the skills of a contortionist will help. Just a word of caution, don't think you are going blind if you can't read them easily. Some were stamped perfectly legible, and some you couldn't read with the engine out of the car.
Good luck with your mission! :classic_smile: