Having owned MANY of both types you are considering, let me make the following observations to you:
81-83 regal vs 84-87 regal have two big differences for a car you're looking to build, and assume many drivetrain changes are coming.
1) interior/exterior updates. 1981-1984 base regals had interiors that the fabric wore better then the 1985-87 (excluding limited which were the same in both groups.) The 1984-down used a pallex style cloth like the grand nationals. In 1983-down you got brushed stainless gauges that were square pods, and, 84-up an ugly (imo) sweep style block. 83-down used a different header, more chrome, more squared off. 84-up more streamlined. Taillights switched but can be changed in about 90 seconds per side, it's all hand-tight plastic wingnuts and press/snap light sockets.
2) mechanical systems. 1983-down systems were more vacuum and solenoid operated as opposed to electrical, especially things like cruise control, your wiper motors and such were different, so what you re-use or need to modernize somehow will vary.
OK your cutlass, you've got a different split, basically 81-86 and 87-88.
Difference here was you never got the pallex cloth, but, 1986-1988 had a nice wearing ribbed cloth.
1987-88 all had the European style header panel, and, parts are pricey for them as people like the cleaner streamlined look.
Same 1983-down and 1984-up differences in vacuum, cruise, wiper systems as the regals, but much like the regals, I Omit the drivetrain as I suspect you plan on major changes there.
Generally by mid-year 1983 you could see frames start to have a second set of holes for alternate transmission crossmembers, thans to the 1983 t-type rolling out the 200-4r.
Of the two cars, if you plan on taking apart and putting together the bodies the Regal platform is MUCH more friendly to work on, fasteners are more easily accessible to remove filler panels, header panels, taillights, etc and there is less to them.
Likewise, the 1985-up red/maroon cars had fabric fade BAD compared to the colors used 1984-down, across all the car lines.
We could talk in depth about year specific and mechanical nuances, but, may make more sense to wait until you pick some specific victims
