How did you drive those first 2000km? Long drives? short trips to the shops?
I take it you drove it pretty easy after the last rebuild
If there is oil in the exhause from the blown motor and you have been driving it pretty soft for the past 2000km it may be that the oil has only just made its way to rear of the exhaust system and started spitting out.
Has it done it the whole time since the latest rebuild? the spitting?
My car spits black muddy dirt in the mornings when its cold, which is condensation in the exhaust+carbon and shit from the engine running rich. It also blows smoke in 2nd gear and only second gear which leads me to believe its just running rich and its all unburnt fuel.
It could be a head gasket, A mate rebuilt a Fiat Twin Cam 4 times due to a blown head gasket!!! With the Fiat there is not much room between the cylinders and the water journal always leaks if its not seated PERFECTLY. This could be the case with your motor, but you would notice rough idling/running, bubbles in the radiator when engine running.
Whats your coolant level like?
That could be another thing, if the head gasket has gone, there is excess moisture in the exhaust causing more black gunk to spew out the end as its flushed clean.
Something like that would be repaired for free by the builder, especially if they didn't have the head machined before rebuilding.
Don't think the worst mate, its more than likely something simple!
Mimmo