80mm MAF on a Stock 5.0 ??
 

I have a 80mm MAF (for a Lightning), how well will this work on a Stock 89 5.0 w/GT40 heads ??

There is no need for an 80 mm MAF on a basically stock 5.0. You'll probably notice more of a loss in low end grunt than you would gain on the top end. The 80 mm is more practical for blown applications and probably not needed even on a modified naturally aspirated set-up.

Plus, you would still have the TB and intake as a restriction on the way to the heads and would need to make sure your MAF is calibrated to the injectors you are using.

He's right I got an 80mm from a 98 Cobra (for supercharging later) and I thought I'd try it on a stock 95. So I got a MAC cold air intake up to the MAF housing. Didn't notice a thing except that it made my air fuel gauge read on the lean side.