What do you mean by you 'just rebuilt' it? You rebuilt the engine? When the speedometer is reading 100 mph, what are the RPMs? If the engine is running really fast and you're only doing 40 mph, it could be a mechanical issue causing it to slip, or maybe the fluid is not full enough or not in good condition. It could be an electronic issue, though. After the rebuild, when things were put back together, if something wasn't connected correctly, I could imagine this kind of trouble.
What other info do you have? Does it shift nicely up until high gear? And, by high gear, do you mean overdrive or the gear before overdrive?