[Xeen]

All this new information is helpfull, this is stuff we needed to know to help you diagnose the problem.
I'm like you I'm not a transmission guy so I really can't tell you if a faulty torque converter can cause the issues you are reporting.
With your description of the work you have done on the engine some of the guys might have more ideas for you.
It is curious that the problem only presents itself at 45 MPH, I am not a expert engine mechanic so I can only offer limited help, however an experienced old school mechanic should have a very good idea what the problem is after a single test drive, like the mechanic Wayne Carini has on Chasing classic cars.