Jst do what i do, take the smaller tube insert it in the larger tube and cut like 2 inches off the end of the smaller one. Then just use those metal clamps that use a screw driver to tighten with and clamp it shut. Its fairly easy.
To increase my water capsity i used 3/4 and 1/2 tubes, the smaller tube being on the ends of the barbs on my block and raditior and pump leaving about 2 inches of the smaller tube to work with. Then i stick the smaller tube inside the larger one and connect the dots, i then clamp all the connections