To answer some of your questions,some exteranal clues would be a special air cleaner assmbly,the early 73s usually had a chrome lid,the latter ones had a painted top.There should be an extra vacum operated door on the side that lets extra air flow to the carb.It will have the 4 stamped on the corner of the heads next to the front edge of the valve cover.The Harmonic balancer is very thick compared to others.Also look at the altenator drive pulley.It should measure about 3.75" in diameter.It should have a 4 bolt main block(have to pull the pan) The oil pan has special baffeling.It does have special motor mounts as was said,they are interlocked.If it is an auto it will have a c6 with a 10" convertor(when you drive it and get on the gas the trans will almost sound like it is slipping).You should have a 9" rear with a 3:25 ratio with AC. The four speed cars usualy got a 3:50 rear(no air) with 31 spl.axels.
You should also find staggard rear shocks,rear sway bar,etc....
The early 73s also got the through the valance exhaust with the chrome tips,after December they went back to the turned down exhaust like the Boss 351s(this and the non ramair on the Cobra Jets was due to EPA noise regulations that went into effect in January 73)
Ok let me through this out there to I have seen a long time ago some of the publications rated these at 245HP,but I think most would agree that it was 266 HP.
I would say that the 73s are a very good buy!I think that you will find more 73s with Q codes than 71-72s only because Ford had already decided to get away from the Clevelands and was using up their inventory.These were available in74 for the last time in Torinos and Montigos,and they still ran very strong.Hope this helps some. FastE ::
'73 Mach 1[/color]351C"Q",C6'3L25 White deluxe,folddown,gauges,added factory ram air and magnum 500s.Bought Sep.3'1973