87-88 LX 5.0's had the same seats as the 4-bangers....so technically there is zero difference in interior between those cars and the 2.3's.
Swapping in a Black interior is a peice of cake. I put a 1990 GT black interior into my '88 LX. The new dash bolts up fine with the tilt column if you use your original column trim and cluster surround.
the conv seats are different. even if you had a 94+ vert seat you would have to have it modified to fit. the easiest thing to do would be to remove the fabric from the 94+ and install it on your fox cushion. the shapes are similar enough that it should fit without a problem. I used the material from a full sized sn95 back seat to cover the cushion in my 93 and it only required a little trimming on the bottom.
all the interior differences i can think of i will llist.
87-88 ALL 4 cyl and ALL 5liters LXs had teh same interior. had the crappy lx leats. The GTs were the only ones to have the GT seats.
in 89 the 5.0 LX got the gt seats making the 5.0 lx and gt have the same interior. the 4cyls still had the same old int. mid 89 gave rise to the pony emblem on the dash of all mustangs
in 90 5.0 cars got map pockets instead of the carpet like the old cars. and they got the airbag wheel and no more tilt option. for a half a year all cars came with a armrest delete from the factory. also the HVAC knobs changed.
also, the removed the horizontal stripes from the gt seats, front abd back. the back of the seat also became shorter and had a smaller head rest.
in 91 they stopped producing door panels with the cloth above the arm rest and remained vinyl like the rest of the panel.
in 92 all GT seats came with NO leg rest like previous years, and was all one piece. and i think they deleted the map light this year i am not sure. also, they introduced the all one color interiors. instead of the red with black upper dash and bezel and wheel. the whole interior was red, same with blue. grey and black remained the same. and the dome light also went from rectangle shaped to an oval shape