Now, I know anything will fit anything with a little work ::...But I want to know if there is any difference that would make a GT exhaust not fit on a 289. I'm going to be putting on some headers also. Thanks.
GT was a handling package. It had nothing to do with the engine or the car itself, other than the fact that all early GT's were either A or K code cars (289 V8). Therefore, the only thing you have to worry about is the rear brake lines if your car was originally a single exhaust car.
As a side note, part of the GT package was dual exhaust.
FYI, the tailpipes for a '65-66 GT will NOT fit without replacing or modifying the rear valance or modifying the tailpipes with a "turndown" on the end. The original "GT" exhaust exits through holes in the rear valance. Chrome exhaust "trumpets" and trim rings around the holes finish the valance off.