Hi Newbie17:
Are you saying your 87's gasket does not have a groove for the locking strip (trim piece)?
I personally have never seen a squarebody without the gasket groove in the gasket. If nobody chimes in, then you can go to the parts books(on this forum tech section) and see what the situation is.
If you do need to get the trim piece, be aware that I think the trim piece design changed after 1986. For my pre-86 truck they were available in black or silver.
My opinion: the trim piece is a good thing as it applies pressure to the outside of the gasket to seal water and dirt from getting between the gasket and windshield/body.
Regards,
Henry