I am buying a 1999 F150 and the kid through a rod through the block and he pulled the old motor...it's a 5.4L SOHC vin L. Do I need to find one out of another 1999 or can it be interchanged with anything from 1997-2003 as long as the 8th digit is an L in the vin?

 

I'm pretty sure the short block is interchangeable with all years but 97-98 have the older more restrictive heads and intake manifold design.

 

There is a few difference as stated above as well as some wiring differences and vacuum line differences all doable just a little extra work in my opinion just find a 99 if you want it to go smooth