I have a 2005 f150 lariat 5.4 with 223k miles on it. I'm just curious if it is possible I could fit a 12 valve cummins into my truck without changing transmission and everything if I swapped engines would it fit? And what would I have to change? Thanks!

 

Anything is possible with enough time, effort, and cash.

However, I think the F150 of that vintage would be a poor choice to convert. The engine bay never held an engine more than 4 cylinders long, and the front suspension didn't carry anything as heavy as a cummins.

 

It's doable there was a 1997-2003 style F150 where someone did a sas and 12 valve swap. Very cramped compartment and would require "massaging" of the firewall with a big hammer. Not much out there for a swap like that in an F150. Have you thought of other engine options like an Isuzu 4BD series or a Deutz air cooled engines?