first, the whole front clip is a SUPER easy swap… literally like max 10 bolts. i’m sure the rear is just as easy. I always liked the euro style (89 up) headlights, but i agree the earlier taillights look wayyyy better.
i doubt a flush is going to fix your tranny, usually once they start slipping its all over. worth a shot i guess. easy enough to replace anyway.
i agree on the gas mileage. those old caprices suck.