Clearance between the manifold and the final length of horizontal ducting

Hiya,

My 8" build, following the builder's guide, is perhaps halfway done.  One thing that's come up is that I have a little less room behind my manifold than I thought.  My heat exchange ducting runs through around 30' of bench (doubled back) and then comes right behind my manifold before heading up about 6 more feet of stove pipe and then 20ish feet of class a chimney.  The heat exchange ducting, galvanized HVAC ducting, passes close to the manifold (which is half a steel 55 gallon drum).  I have some wiggle room but it's going to be tricky to get more than 2" separation between the two and the run would be straighter with about 1".

What kind of clearances are acceptable here?  Can I add something to the design, eg an insulator between the manifold and the ducting, to help out?

This picture might help convey the situation, though the manifold is not in place (but I'm sure you all know where it goes).  https://imgur.com/ICV4Tb6

Thanks

Hi Jean Paul;     Passing next to your manifold is normal. No need to insulate between them. It shouldn't hurt a thing.