1996 Cadillac Deville Loss of HP and White Smoke

I understand white smoke might come from head gasket and cracked block, but added with loss of hp at around 60 mph is what gets me.
I've been reading a lot on your forums and q&a's but I can't seem to find my problem.
White smoke comes from my exhaust and on the freeway, around 60mph the car starts to miss. When my pop took it to the dealership they tried to tell me cracked block but I thought that was fishy because the car never overheats and there is no fluid loss. Just the antifreeze goes down occasionally but its about normal. I talked to a few other mechanics in my area and they all said the same thing, if its not overheating its probably a different problem. What do you guys think? I'm willing to fix it myself, just gotta figure out whats up and what parts to buy.
Thanks!