Geof, the St. Regis is an R-body. It had a Chrysler twin in it's initial run badged the Newport. Plymouth eventually added the Gran Fury badge. Underpinnings are more along the lines of the B-bodies of the year, with the horizontal torsion bars and the bigger rear, the 9.25, available, unlike the M's.
The plus side is: the availability of some of the upgrades. Just like everything cop package that Chrysler offered, bigger was better. The 9.25 was available, as well as bigger sway bars, bigger brakes, etc. Unlike the M's, too, where the axle ratios were rather anemic (2.94s and 2.26s, being the most common) the cop package R-body was standard with 3.23's.
These cars were great handlers at high speeds, brakes didn't want to fade. The engines and transmission were what you expect from Chrysler but they did have some electrical issues. So you need to be careful of the some of the peripherals associated with the cars.
Happy building!