Iv got a 65 gt with a 302.Its got the original greendot automatic C4 in it.It has been rebuilt about 3 years ago.When I drop it in first and try to hold it in the gear until I want it to shift itll shift into second at 4500.Same thing in 2(green dot).If I just leave it in drive and stomp the pedal itll shift a 4000.Is there anything I can do to keep it from shifting untill I want it to shift other than a manual valve body?