yep, what scott suggested sounds like a good possibility, coolant/water in the muffler. If it is only doing it after you run it for a bit and mainly when you accelerate that might explain it.
I would run it more, at high rpm's too and monitor the coolant level, the temp, the engine oil, all of it very closely while doing so.
If you had a cracked head or block or damaged head gasket you would see the white smoke immediately usually, since there isn't immediate smoke till you rev it up that makes sense to me too, excessive residual moisture in the exhaust system.
I'd say run it for a bit and see if it burns out whatever is in the exhaust and muffler, it should clear up in a short amount of time unless there is a lot of it in the muffler. Rev the engine to governed high rpm and hold it there for a minute and let it off, and do that several times, that should get the exhaust very hot and should burn out all that left over moisture in it.
Not too long ago i did a head gasket on a PSI engine and when i first started it up it did that, i had to run it at high rpm's for a bit to get the crap burned out of the muffler, it cleared up after about 15 mins of doing that.
If you don't notice it clearing up after that then yes, you may have an issue you did not find or may have created during reassembly.