I also upgraded my stock 80mm MAF to a 90mm MAF on my '99 L. Along with the FMS 90mm MAF, I also purchased a new inlet boot so I don't know if the one sized for your 80mm MAF will work.

Remember though, unless your stock airbox has been changed to an '01 and up airbox, or you changed to an aftermarket setup, the opening in your stock airbox going to your MAF will still only be 80mm.

All trucks are different, but you shouldn't need to tune the truck after upgrading to the 90mm. I was told by my local dealership I wouldn't have to, and the majority consensus of what I read at the different Lightning forums stated I wouldn't have to either. I was told the computer will read the 90 and compensate accordingly. Everyone was right. I didn't have to tune or reflash my computer. It took to the 90mm MAF like a duck takes to water, with ZERO problems. No pinging (detonation), idle problems, or MPG issues.

Stewart