Is a 78 Dodge Diplomat considered a B-Body?

Kinda looks like a b-body platform. Later Dippys were F or M body? Thanks in advance, WINGNUT

M body from beginning to end in '89

^^^^ This

M body, which is essentially the long wheelbase Aspen-Volare sedan platform with different styling.

wasn't the aspen-volare an F-body ?

yes

thank you JRR.

Aspen Volare are F body.
Diplomat, Lebaron, Caravelle, 5th Ave, Mirada, 80-83 Cordoba, 81-83 Imperial are all versions of the Aspen Volare. Underneath, they are all the same car.
All the 4 doors are essentially the same car with different taillights and grilles.
An 89 5th Ave M body is just a slightly updated 76 Volare F body sedan.

I agree mostly although I can say from experience in owning both F and many M's that there were quite a bit of changes and improvements. The 82-89 M's for example had much better bumper quality, bumper fills, plastics in general such as grill, taillight quality and many other things such as rust protection too. The major thing they shared was the transverse t-bars.

I would hope from 1976 through 1989 there would be improvements!
Common reason why most people lump these as F/J/M bodies, they basically all share frame rails, floor stampings and firewalls.
1976-1980 F body and 1980-1982 M-body 2 doors share a 108.7" wheel base
1976-1980 F-body 4 dr & wagon, 1977-1989 M-body 4-dr & wagon, 1977-1979 M-body 2 2-dr, and all J body share 112.7" wheel base.
Some people consider the 1981-1983 Imperials Y-bodies. Most people consider them J-bodies because they share the A-pillar. per allpar: "This Imperial was a J-body, the same as the Cordoba and Mirada, but the carline was Y, standing for Imperial in the VIN (they used Y instead of I to avoid confusion with the number 1). Gil Cunningham of the Chrysler 300 Club wrote that "As far back as October 5, 1978 the Y carline was included in the J body area of the [internal Chrysler] Model Chart. The name Imperial was added sometime between January 25 and September 10, 1980."

The 1981 service manual confused the issue somewhat by listing the Y as a separate body from the J (Mirada-X, Cordoba-S). If the service manual was correct, the Y-line Imperial was given a Y-body designation in 1981-83. "