V8-V6 same trans code T

I have an 88 T-Bird with a V8 an the trans code is T.I have a 87 with a V6 and that also has a T as the code.So I take it that one fits the other?The trans in the V8 is going bad.I change the trans in the 87 because it would not shift right.And that was because I had taken the motor and trans out and when I put it back in I did not hook up the passing gear.I was told It would be okay not hook up,I just would not have a passing gear:nono: .So when I replaced it I still did not hook it up and it did the same thing:banghead: .So I hook it up and it worked:rofl:.Now I have the trans frist sitting.Can anyone tell me it why the tran from the V6 won,t work will in the V8?