big ol' ford

hi, we recently converted my motor to carberated and also put a different used transmission (automatic) in and when i put it in overdrive and drive down the road it will not shift out of first gear. when i move the shifter down to second gear manually it will then shift into second but if i then put it back into overdrive it still will not shift any more. it is really frustrating driving around in second gear going thirty five to fourty miles an hour and hard on gas also! please Help!!!! i need a cheap solution