After further checks this morning I think the discrepancy in the angle of the crossing vee is small enough to get away with, once the wing and vee rails are in their correct positions. As you can see from the photo, I used pins to hold the rails in their correct positions and then sighted along the approach rails from either end, as well as checking with the track gauges to ensure the stock rails wouldn't be pushed out of alignment, and it all looks ok. Phew!