You are pretty brave.
have the new core sup' sitting next to you before you get started so you can look and not cut off some thing you shouldn't. There are several ways to cut it off , a drill , drill a1/8th inch pilot hole through the center of the spot weld then use a large bit to cut the spot weld off the top layer of sheet metal , but do not go through both layers of metal. Do all the spot welds like this , then separate the core sup from the rest of the truck with a air hammer. ( wear hearing protection !! ) drill holes in the new core supp in the same locations. Remember to grind the burrs away from the holes and coat all bare metal with 3M weld through primer.
line it up the best you can. make a cross mesurement
from the furthest back fender bolt hole to the furthest from fender bolt hole diagonally. Make both sides the same distance. Run a few self tapping screws into the core supp to hold it in place.
Weld it up ( take your battery out of the truck ! ) make sure you pennatrate both surfaces. Skip around where you weld. Two on the left then two on the right , two up top , two down low , you get the drift.
grind your welds smooth , prime it and then paint it.
Or send it to a shop , your choice.
