The Explorer cam really isn't for performance. It's made to get a very heavy Explorer moving, but not made for higher performance rpms.
I have a Mountaineer GT40-P engine in my bird. I am using
BBK unequal length shorties and use 90 degree plug boots to clear them with no issues. You still need the Speed Density computer.
Mass Air really isn't needed until you get into the bigger than H.O cams. With an H.O cam, it's unnecessary. The H.O cam will give you a lot better performance than an Explorer am will.
If you install the "P" heads on that Thunderbird engine, MAKE SURE to do a piston to valve (PTV) check first !!! You may have a piston slap a valve...or few.
Any of the Fox Stang headers will fit the Fox Bird, BUT, remember this...IF you have a column shifter, you'll have to watch out for the exhaust hitting the shifter linkage. My Bird is a Sport model, factory floor shifter. Now it has a T-5, so, no issues for clearance at all.
Best thing to do is be patient, get a good game plan going, gather the necessary parts that WILL work, and go from there. Getting in a hurry will cause you a lot of problems.
To add to all of that, if you go bigger than an H.O cam, you'll probably need to swap the torque converter as well.
Believe it or not, the H.O upgrade for these cars can be done for super cheap. I know you want power, and have access to the "P" heads and intake, but you'd actually be better off getting the H.O cam, some E-7 heads, and even some 1.7 roller rockers. At least until you can either swap pistons, notch your pistons, or get a new short block.
Can you get the whole gt40-p engine? If so, That's a good start. Swap all the accessories,oil pan, distributor, H.O cam of TFS-1 cam (then you'd need the Mass Air with the Trick Flow cam), and other bits. Ready to Rock...except for the need of a new converter as I stated earlier, and better gears.
I got my engine from a Mountaineer. It was a service replacement engine with less than 15,xxx miles on it. Got it for $125.00. Best money spent on the Thunderbird. I put the engine on the stand, and got busy. I sold the accessories and unused parts from the engine for more than I paid for it.LOL. I had the heads ported, and also installed new Trick Flow valve springs.
If you use the "P" heads or GT40 heads from an Explorer or Mountaineer, you really need new valve springs. The factory springs are way to soft for performance use.
E-7 heads can be found on any EFI equipped 5.0 Ford pickup truck.
My email is up above, contact me if you need help.