That's a good old pump. The plumbing is a complicated mess! I'd simplify the plumbing and replace what I could with PVC. Since you are going to install a larger tank, I'd remove the AVC control and fittings from the side of the pump and plug it or install a street "L" and a pipe plug for priming the pump. Remove the small Well-X-Trol tank and cutoff valve and connect the larger tank in it's place. I'd remove the pressure switch from where it is and plug the hole. Then I'd mount the pressure switch as near to the new tank as possible. I'd use the existing pressure and adjust the long screw under the PS cover to raise the pressure. Be careful not to raise the pressure to the point the pump can't build enough cutoff pressure. NOTE: I wouldn't replace the pressure switch until I am forced too as the replacement switches aren't as good as the old ones! I'd replace the drain connections on the pump and the suction line with just pipe plugs. The more the connections, fittings and valves the more chance for a problem.