Did you put the rear main in or know how it is put in? This is a very common problem on rebuilds and the usual source will be the lack of silicon between the base of the cap and the block or possibly improper installation of the side seals. Some folks say it's taboo to put silicon here, but it's right in the GM service manual for the 3.8.
My personal belief is under boost conditions everyone pressurizes the crank case to some degree depending on general engine condition, ring wear, etc. Without sealant between the base of the cap and the block, oil WILL find its way out here and eventually wind up in the dust cover. Torque converter rotational turbulance will push the oil out the starter hole in the dust cover and get it absolutely everywhere, including drips on the cross-over.
May be coming from somewhere else, but this is a best guess if all connections are correct including the turbo drain to the block. I also use only permatex ultra black rtv in place of the side seals. Its more trouble but guaranteed success if done right.
Might also check rear of intake. If block at the rear is dry, the very likely source is the rear main.
Good luck. I hate oil leaks and these cars will get on your last nerve chasing them.
