Yea, running flowtechs on my 89 with gt40p. Any long tube truck header will fit around the plugs, however....mustangs have the fitment issues because of engine bay space. The truck header tubes inherently "protrude" farther away from the flange before the first bend, allowing better clearance for the plugs. Because mustangs have less engine compartment around the engine, they are more likely to bend sooner and cause issues.

Shorties are a different story, they are much more likely to have interference issues with plugs on gt40p heads, as the tubes are more jumbled up by the plugs and some shorties have tubes that go across the plugs and will not fit altogether. Your safest bet is long tubes, which should be music to the ears as long tubes generate power where shorties do not (shorties don't inhibit power, long tubes *generate* power).