Well, you've got a lot of options available to you depending where you wanted to go with things.
Here's a short list of unique changes I know of between years:
1981 - windshield wipers are dash mounted, not column. Wheels studs/brake hardware is sae not metric like 1982-up.
1981, 1982, and *some* early 1983 frames lack the second set of holes on the frame to move the trans crossmember for a 200-4r.
Once the 200-4r was in use you could order it with the n/a 4.1 v6 through 1984 when the engine was discontinued as an option.
1983 ttype was the first to receive the 200-4r, HOWEVER, it did NOT have an 8.5 rear yet.
1981-1983 cars could have option 5 gauss pod performance clusters. All types got the 5 pod cluster, but swapped in a boost gauge in the lower rh pod.
1981-83 cars that had standard 3 pod gauges still have the 5 pod housing behind the woodgrain bezel, so there is room to install extra gauges if desired.
You know of the header panel and taillight differences from earlier replies. However, if you swap the front header panel out the front bumper fillers need changing as well. (Rears of course do not).
1981-83 had clear front bumper lenses, 84-up were amber.
1984 had a redesign of the gauge cluster, dash wiring, and radio went from shaft style to 1.5 dinn. It is possible to convert between the two styles of radio by unbolting and removing the steel plate that holds the cigarette lighter/radio/climate controls.
1981-83 have the same woodgrain dash plaque over the glove box. 1984 used a raised woodgrain font, 1985/86 used a flat woodgrain. In 1987, all base regals for the same silver/black trim on dash panels and door trim like a turbo car, even on limited models. The 1981-86 cars didn't get that unless they were turbo cars.
Wiper motors changed between the years. Early cars didn't have the delay settings the later ones did, and the pulse delay boards aren't reproduced.
If dealing with cruise control, early cars used a solenoid for vacuum control later cars didnt. The solenoid isn't reproduced, but can be sourced from other cars including cadillacs, vettes, etc.
Rear dash package trays varied by year - 1981-84 were a painted hard surface. By 1985 they were carpet covered. Later, 1986 and up cars, had 3rd brake lights as standard safety equipment with holes in the carpeted trays.
1984-up cars had a rpo sticker on the trunk underside... earlier cars did not.
1984-up turbo cars had a unique wheelwell for the down pipe. And a bonus heat shield on the climate box.
The 1984-up turbo cars had a baffled tank with pump inside, and slightly larger diameter fuel lines. (4.3 monte carlo can be a source of the same)
Landau top and hardtop used different rubber seals on the opera windows, and are not reproduced.
86/87 t tops could be locked with a key, earlier versions couldnt.
1984-up cars occasionally got a cut down front bumper reinforcement (typically called the "export" bumper) that provides weight savings.
Electronic climate control has unique dash wiring, as did the digital dash (available 1984-up both turn and non turbo).
Cars with the cornering lamps in the fenders used a unique front wiring harness, but, can be swapped into a car that didn't done with them with little work.
A 60/40 bench seat car will have the floor mounts in the proper spots for the bucket seats, BUT, the tracks are different.
A limited regal with the 1/2 console will have the mounts for a full length console, just not the hole in the floor for the detent cable.
Concert sound equipped cars had extra speakers in the doors, and, there was an "twilight sentinel" option both n/a and turbo car which would automatically engage lights when the light sensor picked up a dim condition.
If you're planning a driveline swap, cost effective would be finding an 84-87 shell, look for FE2 or F41 on the rpo tag to get the upgraded fast ratio steering box and suspension. Odds are you want better sway bars/shocks to start with. $ for $ you're better off buying a new 9" rear than overpaying for the "ttype/gn" name and getting a worn 8.5...
In 1987, there was a way to order a regal "T" and it came with a 307 engine, but the same interior/buckets/rims/trim as a turbo car. That's why 1987s were "turbo ts" or gn/gnx.
That's should give you a good start. I've taken apart a couple hundred of these, so, once you decide on one you want, and are trying to figure out where/how to get things to transform it, either myself or someone else here can point you the right way.
) as well as most body panels including complete doors. Besides the rear frame rails, these cars love to rust under the vinyl top, bottom door edges, on the frame near body bushings especially the one under the A/C box plus all the usual places you'd expect on a 30 year old car. G body's have a few performance shortcomings but most of them can be easily taken care of and have been discussed somewhere on this board. Most everyone here will be eagar to help you out. 
