Vapors or smoke out of the valve cover breathers generally means either a blown head gasket(which shouldn't be the case on a fresh rebuild and not getting into boost yet) or blowby past the rings(which probably aren't seated)...The other one is cracked pistons or holed pistons(which also isn't the case on a fresh motor that hasn't been driven hard)...
If you have the equipment to do it, run a leakdown test on the cylinders and see what kind of readings you are getting...A normal compression test may show good numbers on cranking compression, but a leakdown test will give a better indication of how well the rings are seated(or aren't, in this case)...
I'm getting ready to put my motor together for my 87 T and basically have done the same as you...BUT, I am replacing some unknown brand replacement +.030 cast pistons with TRW/SpeedPro forged, a re-honeeek: ), a turbo crank to replace the NA crank that was in there, and decked the block(because of a blown NA head gasket that was put on instead of the correct turbo motor head gaskets, didn't want to take a chance on the new gaskets not sealing in the area where the NA gaskets blew and I had no idea how many miles I had on it with the fire ring around the cylinders melting out...Hopefully, I won't have too many blowby problems now(I did have some with the motor before the head gasket completely blew out between cyls #1 and #3)...I had some oil dripping from the valve cover breather for some time before the motor came out of the car...
Good luck and let us know what you find out...
If you have the equipment to do it, run a leakdown test on the cylinders and see what kind of readings you are getting...A normal compression test may show good numbers on cranking compression, but a leakdown test will give a better indication of how well the rings are seated(or aren't, in this case)...
I'm getting ready to put my motor together for my 87 T and basically have done the same as you...BUT, I am replacing some unknown brand replacement +.030 cast pistons with TRW/SpeedPro forged, a re-honeeek: ), a turbo crank to replace the NA crank that was in there, and decked the block(because of a blown NA head gasket that was put on instead of the correct turbo motor head gaskets, didn't want to take a chance on the new gaskets not sealing in the area where the NA gaskets blew and I had no idea how many miles I had on it with the fire ring around the cylinders melting out...Hopefully, I won't have too many blowby problems now(I did have some with the motor before the head gasket completely blew out between cyls #1 and #3)...I had some oil dripping from the valve cover breather for some time before the motor came out of the car...
Good luck and let us know what you find out...