Automatic transmission slipping in lower gears

Hello,

I have recently had an engine swap done on my vehicle listed above it is the Classic model. Before the swap my transmission was fine however after it seems to be slipping when going into higher gears. I have checked the ATF dip stick and it appears to be reading higher then expected. I am hesitant to add more fluid as I do not want to overfill. I plan to take it to get looked at in the next week or so. Any advice helps.