stock lower 90 to stock lower truck
 

does anybody know how well the lower truck intakes flow as compared to my stock lower one?

I actually have a truck lower intake in my shed.....not that i'd use it. I've heard they flow better, but not a whole lot better. If you look at a truck lower they have the square ports, but just like the normal mustang intake the runners don't distribute flow evenly especially to cylinders 1 and 5 because of a sharp turn on those intake runners.
If you want to get a relatively cheap intake you could probably pick up a '96 - '00 Ford Explorer 5.0L intake manifold upper and lower (basically a Cobra manifold) at most junkyards. They flow i believe 195cfm per runner or something close to that and only flow very, very slightly less than an actual ford motorsport Cobra intake. I think they also come with an opening large enough to accomodate a 65mm throttle body.