Does anyone know how i can change the pick up tube on the sending unit of the gas tank from 3/8'; to 1/2';tube

there are a few ways you can do this. All of them actually require removing the tank to do so. The first is to remove the sending unit and then cut off the old 3/8's pipe above and below the mount tab. then you can drill it out to the larger diameter pipe that you want to use. Slide your new pipe in to the correct depth that it needs to be. If you know how then sodder it in real good and let it cool off, reinstall it and make sure you put a good fuel filter in line somewhere. the other method requires a little bit bigger hole to be drilled and an AN fitting to be installed then attache the correct diameter pick up for the intank and the correct diameter for the fuel feed, also installing a good fuel filter as well. Be prepared for the high price of AN fittings and line cost if you go that route though. I have done this swap both ways and find the AN easier to due to avoid leakage and promote actual safty. good luck and hope it helps you some.