Exhaust Woes

I've been battling with my exhaust for about 6 months now and I am running out of ideas. First, somehow an exhaust smell is leaking into the cabin, sometimes through the heat and other times it just seems to find its way in on its own. I've been through the whole things about a dozen times and everything is tightened down as much as possible with zero leaks that I can hear/see. The second issue I am having is that my exhaust is leaving marks at the exit of the tailpipe on my rear bumper. I've had tuned and re-tuned to insure that it's not running rich, but nothing seems to work. Is there something that I could be overlooking causing my exhaust to be exceptionally dirty? Thanks in advance for your help.

My o2 sensors are less than a year old. I'm not too sure about the AFR's though, I will look into it. I had Paul's Automotive Engineering do the Head swap and they removed all of the EGR and the smog pump, I just assumed they buttoned everything up well. What exactly should I explain to them to look for on the heads?

I checked the back of the heads and I think one hole might be plugged and the other might not. I'm going to try and have the shop that installed them take a look at it. Now if I end up sealing them and the problem still exists, are there any other possibilities for the smell?