340 Engine Shake

That's the thing, there are no visible signs. I did a data log and nothing changes. No dips in vacuum, AFR, rpm, nothing. I also used the large Guages feature to monitor that while it happens and no changes. I don't think I have any exhaust leaks. I used remflex gaskets on headers and collectors, everything else is welded. What is a way to check check for exhaust leaks?

Have a friend hold large cloths/rags/plugs of some sort, and close off the tailpipes, you move around listening for leaks, they will be quite aparent.
Caution, be quick, those cloths get hot fairly fast.