Sway Bars for-r an 89 GT

well we can use 79-04 i believe rear swaybars, so you can play with plenty of OEM Ford rear bars from the junkyard

im currently using 98 GT rear bar its slightly larger in diameter than the factory 89 bar

19mm to 21mm/24mm

the front swaybar on an 89 GT is MASSIVE adding to the already problematic of understeer so i do not reccomend going to a stiffer or larger front bar,

however it also depends on what the rest of your suspension is...