losing antifreeze after rebuild

I. Don't really see any major smoke out the tailpipes but I've only driven the car at night. I did see streaks in the oil I drained after 75 miles post rebuild and also have white paste in the oil cap.please tell me it may just be the intake manni gasket. I'm going to pressure test the cooling system and do a leakdown test tomorrow. The car has run very good with plenty of power but just driving 20 miles to work im down about a quart of antifreeze.I would just like to pinpoint it the best I can before tearing it down.

Well after driving home from work last night, it was the first time since the rebuild (100 miles ago) that there was no white paste in the top of the oil cap.

Today I pressure tested the radiator and it appears to be holding 10 PSI for 4 or 5 minutes so im thinking the streaks in the oil must have been a by product of the rebuild, maybe condensation was getting past the still unseated rings . I hammered into the car last night and it ran strong, much quicker than my PI 96 gt did.