I just parted out this exact car with very few exceptions. With regards to the rearend question... When a performance 1973 and later A body is equipped with front disc brakes, it has the 8 1/4" rear, which is what this car will have. A quick check would also determine if it is a suregrip. Being that it is an automatic car, it should also have the SB727, so a quick check under the car at the pan will yield you this information. With regards to the engine, a quick check on the front of the block under the drivers side head will indicate whether it is a 340 or not. Also, check for the cast crank dampener, as a 340 from 73 will have a cast crank, and a 318 will not.
If all 3 of these drivetrain pieces are in place, or at least the trans being a 727 and the motor being stamped 340, I would venture a guess that someone along the way swapped out the 4 bbl carb and intake to save on fuel during the "high" gas prices and heavy inflation ladened 1970's!!! That may also be why it was mothballed, running a 318 intake on a 340 isn't exactly port-match heaven!! If the motor is stamped 340, I would almost bet my bottom dollar that the drivetrain is all original.
As for value??? I paid $1500.00 for a complete, rusty hulk of a parts car with almost the same options that yours has, except for bucket seats and a console. The car here in question should be priced between $1500.00 and $2500.00 IMO. If it was running but not yet roadworthy...$3000.00. Good luck with the deal, I hope you get it!!1 Geof