Antifreeze on my floorboard.

[Pl4yboy]

Okay, I just pulled the original 350 I blew ad put in a brand new crate. I never noticed this before this happened, but now I have antifreeze leaking on my passenger floor from inside. It seems to have two leaks, one from the firewall and one from a missing screw on the plastic. I noticed once before (about this time last year) that it there was smoke coming out of my vents when i turned on the heater. I drove home and by the time I was home, it was fine and never had the trouble again. The truck sat for a month and a half before I got the motor in and now it's doing it very regularly even when the heat isn't on. There is a very nasty oily film popping up on everything after it happens. I didn't touch the heater core or anything during the swap. The motor side of the firewall is fine too, with no leaks. I'm thinking this is either a bad heater core or something is leaking on the interior. Is there any possibility this could be the A/C? I'm doubting that. Any ideas guys? I can get pictures any time.