Sorry man, with out any more info it is kind of hard to tell.
I think a stock 88 GT had 205HP, then if you add a cam you might get 10-15 more HP, and the mufflers probably won't make any kind of difference (a couple hourse at the most).
Now if you have an H-pipe or X-pipe, that will change things you might get another 15-20HP again.
Hard to say, with just the cam maybe 200 RWHP (rear wheel horse power)?
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1995 Mustang GT