What you are experiencing is extremely common on rebuilt motors. When they reworked the head, the valve guides were worn. They took the cheap way out and knurled the valve guides. This lasts for around 10,000 miles in stock engine. With a higher lift cam, they last even less. The guides get worn and under high vacuum situations, oil gets sucked by the guides and burns as smoke. It is most normally noticed after driving and getting it hot and then sitting idling in a fast food line. At an idle, the vacuum is the highest.
A bandaid can be to replace the valve seals, but normally you need to pull the head, have the valve guides sleeved with brass/bronze inserts and then reamed to size. Check you engine warranty. It should cover it, but if they find out you are running a hot cam, they will most likely void your warranty.
This is why I preach against getting a crate rebuild. A long block is fine, but never get a rebuilt head. Have your old head redone locally where you can guarantee that the guides are sleeved, not knurled.