"My XR7 2004" If you really want to go for the larger MAF you can, but it is my understanding the stock MAF does not really act as much of a restriction on a relatively stock 3.8 MN12, and actually flows enough air for your current requirements. I believe the TB, further down the air intake line (along with the heads), acts as more of an air restrictor than the stock MAF. You may want to start there first (if you are considering half-shafting your TB check this link out and be careful http://forums.tccoa.com/showthread.php?threadid=168). Someone correct me if I am wrong, please!
That's what I was told by "The Man" about a year ago. The MAF on a 3.8 is not a restriction like it is on the 4.6, he told me he felt the LMAF was a bit much. On the TB, half shafting you'll flow more air....that's the whole idea behind it. To go to a larger TB I think the intake needs to be opened up, plus you'll need the mods like P+Ped heads and intakes plus a cam to make it all work.