water in oil after heads swapped

all of these are good ideas BUT!
You said that you removed the a/c compressor right?

Could the coolant passage in the timing cover have a broke or cracked gasket were the cover bolt up to the block?

If so it could run coolant right down the from of the block and into the oil pan.

start by re- torquing them gasket. could you have to long of a bolt in that area that bottomed out before it got tight?( water pump bolts)