08 crew cab 6.2 problem

I have an 08 silverado the dod ate a lifter so I decided to swap out the 5,3 for a lo mile 6.2 with a small btr cam .The truck ran for a few days with the 5,3 computor than just shutoff ,Long story sat in shop for few months and finally got it running with a 6,2 computer ,the 6.2 had a broken tb when I bought it so I swapped the 5.3 tb to it and it runs but hard starting now almost sounds like the timing is to far advanced ?drives ok but starting it has to crank 2 or 3 times with your foot on the gas to start and run anybody have any advice? as ton why?