Replaced starter and alternator on my F150, but still having troubles..

Saying you work on chevys and not ford, what does that even mean. Did your ford not have 4 wheels and an engine, like the chevys do. Your issue maybe multiple things, a long crank before starting can indicate bad reading form a mas air sensor. Or bad injectors, high resistance in the starter circuit, the other symptoms could be loose ground circuits or a number of things, u need to do basic tests that apply to all cars, like checking the amp draw while cranking( using an amp clamp) on the starter red wire. Check charging voltage, hooking up a fuel pressaure gage and see the fuel psi while cranking, and other tests used to diagnose a starting concern

The main ground wire for the truck goes to one of the starter bolts. The cable connector on these cables are thin and weak. If it is not clean and tight. The starter will turn slowly and cause all sorts of strange electrical issues. Also make sure the battery terminals are clean and tight.

Start cheap. Try a 7 dollar relay before you drop 200 dollars on a guess