2000 GMC Sonoma Engine change gone bad

He's the story, I've just recently exchanged engines in my truck with one out of lower milage S-10, the engine that was in it was running really ruff and White smoke was pouring out the exhaust. The engine I replace it with is the same type and year and everything match up exactly. After getting everything put back together I started it up and it started running almost identical to the way the other engine was running, Really ruff, missfiring, and White smoke pouring out the exhaust with a strong gas smell. So we changed both O2 sensor and gas filter. It started to run a little better but still pouring out the smoke and it's missing on only one cylinder also I noticed an oily gassy smellying leak comeing from somewhere around the top of engine around the exhaust manifold. Not sure what it is or could be because i've never seen it before. This is getting frustrating, I've been working on it for the past 3 or 4 months now just to get this far to find out that the engine I put into the truck is bad. Any idea's or sudgestions would be helpfull.