if the mounts on the top of your mark VIII shocks are still in good shape, you can reuse them on the t-bird shocks, i use either monroe shocks or gabriel's.
if the rubber bushings in your stock mark VIII top mounts are bad, take the stock shocks out, take the 18mm nut off the shock rod and remove the stock upper mounts, cut the bushings out of them, usually a mapp gas torch will work, and then use the new rubber bushings that will come with the new shocks, i have done this a dozen times on different marks, works perfect, there is no need to buy any new upper mounts, just reuse the stock ones and do what i said above, cut the old bushings out and use the new ones that come with the shocks. gabriel's are on sale at autozone, $24 bucks for a pair. how can you beat that!!! and they ride great.