Th350 feels like it's slipping

[Mike harris]

Hi guys, just rebuilt my 350 sbc into a cheeky little torque monster and done about 700 mile so far, engine is running like a trooper. Buuuut it now started slipping in second and third in the box. If I'm in 3rd and it kicks down the revs pick up and then slow down and it starts to pull a lot harder when the gearbox actually settles down. It's had a new filter and fresh Dextron 2 oil in and the level is perfect. Could this be worn plates or bad adjustment on the kick down cable etc. I'm unsure on the correct setting for the kick down cable on our trucks could anyone spread some light on how to adjust it properly?