Is it possible to put a manual transmission in my
Yes, it is possible to put one in. You will just have to put the stick up in ur cab somehow.
I am pretty sure a 6 speed was an option for your truck. You would just need to source all the needed parts. I suspect you would need to re flash the ECU to work with a manual transmission.
Possible but not practical due to all the parts required.
I wanna rebuild the truck into a diesel what engine would be best for it
Get a diesel donor truck but you would be better off selling your truck and buying a diesel.