The last B-body 4-door was made in '78, with the 2-door B-bodies ending in '79.
The R-body is a re-skinned B-body 4-door with production from '79 to '81.
The entire front/rear suspension, including K-frame and differentials are exactly the same for all B/R-bodies made from late '73 to '81. All rear suspensions from late '73 and on, used ISO clamps with the exception of
some early police cars (which didn't use ISO's).
The larger 11-3/4" brake rotors are from '75-81 with pin-type or slider type variations (FMJ's use sliders).
The '66ish to early '73 B-bodies (and E-body) used a normal looking K-frame (worms eye view):
The late '73 B/R-bodies used this design (and also used the same front K-frame rubber mounts that FMJ's use).
Note: this looks more like a X-frame and is heavy. It takes two good sized guys to pick this thing up.
For comparison, this is a C-body K/Sub-frame:
There is enough metal in this to make a new car.
On a unrelated side note: I ran across this:
K Member Supports | X-Parts X427-1473-S
The red X is for the separate transmission frame. I "think" the '73-74 used slightly different mounts (same for all 4 corners, I think) so don't think but two will fit our cars - but it makes me wonder...
Nope, solid (iron, aluminum or urethane) is the only way to go!
BudW