The '66-67 rear wheel well is very restrictive, on a '66 Belvedere 4dr winter beater I had once a factory 15x8 Ramcharger/pickup wouldn't work on the rear, and just as a guess, the wagon openings aren't much different.
The '68-70 well is much bigger.
I am guessing from the look you have the late '80s M body, 15x7 wheels on there now. Dippy cop car wheels are identical but with slots.
