Have a 68 GTX, trying to find out if the trans and rear end are original.. Its an automatic 440 car.
Where would one look and look for?

Thanks

8 3/4s didn't get stamped with any #s like the engine/trans did. The 8 3/4 used paint stamps and paint markings on the housing and center section. If the housing has been blasted and painted before(or if it's super rusty) those paint marks are gone. You can soak the housing and center section in a rust removing product like EvapoRust. It will remove the rust and leave behind any leftover paint. You might get lucky and still see the factory markings.

And if he is able to uncover original paint markings that would help to determine if the rear is original how? Answer.....it won’t. Best one can do in this case is to determine period correctness.

Trans will have the vin stamped on the upper lip of the bellhousing( same area as the engine block )
Look in the area where the oil pressure sender is

rear axle will not have the vin,

Correct, however if all the assembly dates are uncovered and correspond to a narrow time frame around the SPD, then chances are it's the original. Very unlikely that a replacement housing and center section were installed with assembly dates the same as the factory original one.

Take a look at the front pad by the distributer and post a picture or post what is stamped on the pad. Remember B 1966 C 1967 D 1968 E 1969 F1970 G 1971. The other numbers and letters on the front of the block and pad will tell some of the story. _440 HP2 Cubic inches, High Performance. Other markings will tell a story also. If the marking on the pad state it is a 1968 Motor then look at the stamping at the back of the motor and top front of the transmission and see what serial number is in that place. 1969 and up have the VIN on the side of the block on the pan rail and even then it is either a full or partial VIN depending on the year.

The 8 3/4 housing will have a casting date by the casting number on the housing, and if you pull the center section out the gears and sure grip will have a date stamp on them.

I was under the impression that a 68 model year was a hit or miss deal on the VIN stamping, IE some had it and others used the BSO # (build sequence order) found on the build sheet / body stamping s etc.