There is very little aftermarket intakes available? There is an updated intake available from Ford and
parts stores that has an aluminum crossover. This is a direct replacement intake and is rather costly.
Performance intakes for the 4.6 is very limited. There is only a few availabe from after market companies and they are very costly. The only one that is somewhat reasonable in price is Fox Lake's P-51 intake(
www.foxlakeracing.com ). I am not sure this will fit under a TBird's hood.
Now for what Ford offers, Ford offers the FRPP intake( similar to 2001 Bullitt intake but works with 96 - 98 heads and FRPP Heads ) which cost about $1600.00 and the install kit is about $300.00, similar to the FL P-51 in price. This intake requires some EGR modifications and Hood Bracing to be cut.
Also, the 99 - 04 4.6L Mustang GT intake will work. This intake is cheaper than the replacement one for the 96 - 98 4.6 and is a lot cheaper. The intake and
parts can be had new for around $200.00 if you have a good Ford
Parts person, I can recommend one as well. This intake breathes a lot better than the 96 - 98 intake, dyno graphs have shown about 15 HP increase across the board and 25 - 30 HP increase at around 6000 RPM. However it is not a direct replacement.
This route I recommend you get an 01+ intake as it has the aluminum crossover( won't crack ) The 99+ mustang only uses one sensor while 96 - 98 4.6L have two in the manifold. You need to drill and tap a hole ( 3/8 NPT )in the Aluminum crossover, this is the easiest solution to the two sensor problem.
What else you need to do is the following. You need the coolant tube from a 99+ mustang, the one that goes in the valley underneath the intake. If you have auto climate control, you have to modify the coolant tube to accept the sensor that is on the 96 - 98 tube. You need a 99+ O-Ring for the water pump and the alternator bracket for the 96 - 98 intake with Aluminum Crossover. The 99+ mustang one, will not work and the 96 - 98 one for the plastic crossover will not work. Also you will need to use either 96 - 98 intake gaskets and use RTV around the intake ports to seal it or 99+ intake gaskets and RTV around the coolant ports.
The Bullitt intake will work as well but you need to do the mods that the 99+ intake and FRPP intake require.