Here's the vin breakdown of my 78 sedan

8  year 1978
T  Assembly plant code (Metuchen)
10 Body serial code (sedan, a wagon should be 12)
Y  Powerplant (2.3)

The six digits following this is a sequential serial number i.e 183742 is the 183,742nd car built.

On the inside of the driver door there is a tag with other build code information, like interior trim, exterior, transmission, and axle ratio.

I just did a reply on paint codes, here are the interior codes and descriptions for 78:

DR    Interior decor group Alpine Cloth(plaid)/vinyl.
HD    Interior decor group Ruffino Vinyl.
GT    Standard interior Kirsten Cloth/vinyl(a no cost option)
AB    Standard interior Colton All Vinyl.
JA    Pony interior Compton Cloth/Vinyl (a no cost option)

Interiors were available in Wedgewood Blue, Bright Red, Medium Jade, Chamois and Tangerine.

The Interior decor group added low-back seats, woodtone appliques, a 'custom' steering wheel, deluxe door panels, color coordinated seatbelts and more.

The Standard interior featured High back bucket seats available in all vinyl or vinyl with cloth inserts, a color coordinated dash and steering wheel.

The Pony was available with the standard interior or for no extra charge, a durable cloth/vinyl option was available. The Pony came with a black dash and steering wheel.

My brochure doesn't show any way to identify the rally package, although in the brochure, it's referred to only as an 'appearance' package.

Hope something here helps.
Chuck