727 help

I have a 727 transmission in a 75 scamp with a built 340 it has a reverse full Manual valve body it was running good then I changed transmission fluid and put atf+4 in it now when driving sometimes it feels like the car is braking and holding it back I can floor it in 2nd gear and it want tach over 4000 and the brakes are not sticking pretty sure it's the transmission. Can someone help if the transmission can cause that