Weird,because when you buy aftermarket upper and lower rear control arms,they are for '79 to '04?! I know the ones i installed last summer are for '99 to '04,maybe the bolt sizes are different (the diameter of the bolt shaft) ,cause i know they are the same length.
The 79-98 arms use 12mm hardware. The 99-04 arms use 14mm hardware. This means the thickness of the bolts is larger on the 99-04. Stores advertise that you can use either arm on any car as long as you can get hardware. This is partialy true. The upper torque box will accept any hardware. The lower torque box does not. I put 99-04 HPM Mega Bite Jr's on my 81 Mustang. I had to enlarge the hole on the lower torque box, then it went right through. So basically, I would not use older arms and bolts on the newer car, because you don't want any slop in the holes, because it will chew up the metal on the body.
That is what i figured.I did them in my '85 GT i use to have,and had no issues with the bolts,but with my '03 GT i did,so what i did was drilled ou the metal sleeves from the stock lower control arms,and slid out the metal sleeves in the aftermarket lower control arms,then enlargened the holes in the bushings in the aftermarket control arms to accept the stock metal sleeves,and used my stock nuts,and bolts this way i wouldn't have any slop to damage the stock sheet metal and holes on the chassis.