1965 Dodge Dart high RPM issue

Ok....so I’m new to the Mopar world have i have been able to figure out a lot of things on my own and some manuals...So I bought a 65 dart 270 and I have rebuilt the motor 225 /6 this baby runs great.....my issue.... I took it out for a spin the other day and I was very happy...the 3 speed auto trans shifts real smooth no slippage I can feel.....But in third gear at about 45 MPH my RPM's are crazy high I don’t have a tachometer yet but I’m guessing like 4000 RPM’s……I’m lost on this one and could use some much needed help…..

Thanks in advance

:banghead::banghead::banghead: