Merely a suggestion.
I would look at all the Buicks from 81-87. The main shell is the same on these. 81-3 front header panels & bumper fillers are unique to those years while 84-87 are unique to those years.
Keep in mind doors, fenders bumpers, Quarter panels, hoods, Trunk lids are al the same. rear fillers same 81-87.
The nice thing about Buick is that they had some cool paint schemes. 2-tone designer series to wh-1's with Black Gray & black similar to an 82
Grand National. solid colors lie rose wood etc..
On the interior the dash pads are the same. 81-3 has a different cluster set up then the 84-7's.
82 you could get a true bench seat car.
If you want buckets or split that is no problem they make mounts for a conversion.
If you want to do a floor shift it isn't that difficult. Just don't do the Grand national clone. If you try & do that you will be better off just buying one as a starting pint.
Just make it your own. Even some GN guys have lightning rod shifters from a
H/O.
Figure out the color combo you want, what HP you want & I'm sure you can find something out there that was produced close to what your looking for.
81-83 parts are generally less expensive then the 84-7 stuff. If it can be sold as a GN part the prices go up.
Does the regal have any roof options like T-top or moon roof?