I'm going through the traps at 106.3 MPH with a TH400, 3.73 rear gear, 28" tires, and the tach is showing about 5100 RPM. The best I can figure, I should be going faster or I'm loosing about 400 to 500 RPM to slip somewhere. Is this normal, and is it acceptable? If not, how does one pinpoint the source of the slip (converter or trans)? Thanks.

If it's your transmission slipping - you're in trouble. If it's the converter slipping - it's normal. 10% slip isn't out of the ordinary.
Transmission slippage will produce other side effects, like burnt fluid and a continually degrading performance.

Its about 350 RPM which is pretty good as long as its the converter.

I don't think you'll find an auto that can slip continually without quickly burning up the clutch pack or band that is slipping. So, the most likely culprit is torque converter slippage and it will always have some unless it's a lock-up unit.

Peter