Ford never put in a 390 in a 4x4 F100, F250 or otherwise from the factory, only 360. The only way yours is a 390 is if a previous owner made the change. Is the 360 worn out? If not, I'd go ahead and do exhaust and intake/carb. Converting to 390 requires acquiring and installing 390 crank,rods and pistons. If you get in that deep, you should be doing a full rebuild. All the exhaust or intake upgrades will still be good when you make the leap to 390 (or 410). Not sure what your budget is, but a proper rebuild is a signicant expense.

 

Just crank and rods, pistons are the same.