I recently bought a 78 DT400, looks almost stock. It is kinda loud for the street, I bought some exhaust fiber filling, the old stuff was completely packed. how tight should I put the glass fiber pad in, it was too thick and made it thiner and got it in the end of the exhaust but didn't seem to make much difference, too tight? 

Maybe the spark arrestor had been knocked out. I'm guessing a bit but removable end mufflers usually have one spot welded deep in the tail muffler if you pull the part out you wrapped in fiber & look deeper it should be visible--if still there. As i say i'm guessing a bit as not overly familiar with those models but talking generaly.

Also the complete chamber main exhaust should have fiber packing held agaist the inner walls by perforated steel so might've let go over the years. That stops resonating noise through the walls but i guess helps a bit with exhaust noise out the tail pipe too. Gets oil soaked & smokes forever unless you go for a very long ride on the road or burn it out.

Compared to modern, the 1970's bikes are definitely loud.

Stock air box made a huge difference so if someone switched to a clamp on filter that could be a large part of the noise. 

Euro bikes might have had welded mufflers?

USA bikes all had removable inserts… with packing on later models.  This was required by the Feds as the USFS spark arrester is required by law here to come apart for cleaning. 

I see the person who I was responding to deleted their post that said USA bikes never had replaceable packing and that the Feds required mufflers/spark arrestors welded up to prevent tampering.