End of a long straight, rear wheel lock up, sounds like a sieze to me.

I don't think you have enough lower gears to lock up the wheel, but it sure would make the motor sing.

You haven't been inside the top end yet so before it spits you off in a turn a tear down is in order I'd say. Hopefully you didn't hurt it too bad and a hone and clean up might be all that's needed. A carb tear down and clean wouldn't hurt either.