Engine problem and oil leak after rebuild

What is your question?

I used to be a professional mechanic but it's been a lot of years since I did any work and I'm new to the import scene. Anyway, we bought a 1995 Mitsubishi 3000GT, SL to use as a show vehicle for our vehicle wraps business. I recently overhauled the engine and got it back together and in the vehicle and it started up fine and ran well for about 4-5 minutes and then stalled out, but while it was running it was pouring out white smoke, which would indicate a blown head gasket. After removing the heads again, I saw no sign of the gaskets being blown but the center cylinder in each head was black, while the others looked normal. Any suggestions would be appreciated while I'm waiting on the machine shop to tell me something. Additionally, at first, I had no leaks from the vehicle, but after keeping it running after it wanted to stall, I saw a puddle of clear oil under the car which stopped as suddenly as it started but I don't know if it's the tranny or the engine so I'll have to pull them apart to find out I guess but if there's no tell tales I don't know what to look for.