Need exhaust expertise

[cj847]

I have an issue with my exhaust that I think I have figured out but want to see if anyone agrees.

1964 GMC Suburban, 2.5" dual exhaust with reasonably free flowing mufflers. SBC engine. Quadrajet.

My problem is when I start it after it has sat for a few days it spits out a ton of water mixed with black goo. It is ruining the floor of my garage.

Anyway, I assumed it was condensation. So, I went underneath with a drill and drilled a hole in the back of each muffler. But, while I was under there I noticed that it was not the lowest point of the exhaust. The lowest point is about 1 foot in front of the muffler.

My question is: Is my only option to take it to a muffler shop and rework the whole exhaust system so that the back of the muffler or further is lowest?

Or is there something else that could be causing this?

Thanks for any comments/suggestions.