I'm having a similar problem with the GTV I've just rebuilt. First of all it was burning oil - which turned out to be the inlet gasket not sealing. Some black-sticky fixed that.
Secondly, it was still burning a bit blue, so Smaky said to remove the exhaust studs and seal them up, which I did - oil was passing the studs, and getting into the exhaust manifold.
Now no more oil, but it still seems to be blowing clouds of white out the back when given a bit of throttle. Compression is excellent, no signs of escaping combustion gasses anywhere, isn't using coolant, and it runs sweetly - save for a misfire at around 3000 rpm that I can't track down.
I'm a bit stumped by it
. All I can think of is that there's a lake in the exhaust somewhere and it's taking a loooooong time to clear.