4-cyl K-members are the same as the pre-1988 V8 k-members. Starting in 1988, the control arm mounting points moved out about 1". It is common to add the SN95
control arms to correct this on 4-cyl cars with the original K-member.
Yes, the cutoff is 1988, not 1987.
There are also some subtle differences in K-members. The V8's used welded spring perches while the 4-cyl use stamped.
'79-'87 Mustang K-member (and all 4cyl) front A-arm bolt center to center - 22.75"
'88-93 Mustang K-member front A-arm bolt center to center - 23.75"
Mustang SVO K-member front A-arm bolt center to center - 23.00"
'79-'87Mustang K-member (and all 4cyl) rear A-arm bolt center to center - 30.125"
'88-93 Mustang K-member rear A-arm bolt center to center - 31.125"
Mustang SVO K-member rear A-arm bolt center to center - 29.5"
'79-'93 K-member angle of A-arm off centerline of car - 16.5 deg.
Mustang SVO angle of A-arm off centerline of car - 14.5deg.
'79-'87K-member overall width from ball joint to ball joint - 51.37"
'88-93 K-member overall width from ball joint to ball joint - 52.37"
SVO K-member overall width from ball joint to ball joint - 53.36"
Stock Mustang A-arms '79-'93 - 13.00" long
Mustang SVO A-arms - 14.00" long
1987-1988 Thunderbird A-arms - 13.75" long.
Sn95 A-arms - 14.00" long
That info was taken from this book