the suburbans and crew cabs stayed square body till 91 also.Old-Topic but maybe this could help..
- 1973-1987 OBS GM pickup (C/K 10,20) were for the most part all alike with the exception of engine and axle options, Updated accessories and trim etc..
( however the K30/K35 or C/30 ran up to 1991 and had the OBS with the newer TBI engine and Trans. options from 1987-1991)
- 1988-1998 was of coarse the IFS era except for the K5 Blazer which was OBS from 88-91.
You should be able to source your system using the 1987 or even a 1991 listing for anythign that was C/K 30. I am not sure of the exact frame differences with the P series Frame but I'm fairly certain a small amount of modification would make it work for your chassis!!