Stock DT exhaust is an expansion chamber.  Later models have a better design than earlier models.

(An exhaust system doesn't need to be made out of rolled sheet metal cones to be an effective expansion chamber... rolled cones are just a cheap and easy way to build a pipe.)

Looking back on your earlier posting , you described thr RZ pipes as pipes and muffler .
I'm guessing now that you are getting late model RZ pipes with detachable muffler canisters .
My experience is with the earlier pipe without a detachable muffler .
They had internal tubes  running between internal baffles . Quite an unusual setup .
I can't really comment on the internals of later pipes , as I've never examined them closely .
They may work fine for your purposes . Best of luck .

Yes i was talking back a few posts about the model without detachable mufflers just the baffle that slid in the end as normal.. They run a tapered header pipe too. A catalytic type exhaust was mentioned so i don't know--we never had to worry about that problem in NZ back then.

These are the detachable muffler style. I will see how they go then report back.
 

My RZ250 exhausts showed up. I brought the left and right but will only need the right side. 
First thing first, was some degreaser and cleaning. Then I opened up the mufflers. Different baffles! Not sure which one I will use yet. I was expecting to see some packing material in their?
The ID of the original pipe at the cylinder is 42mm, the RZ is 32mm. That will certainly affect gas flow.
I lined up the chamber on the bike and it has the look I am aiming for 
The rear mount will need to be moved and attached to the frame somewhere near the foot peg.
The bend of the RZ expansion will need to be modified to meet up with the cylinder. 
Need to work out how to do this!

To get the bend right on the exhaust, I am thinking about making 3 cuts approx where the chalk lines are. Will only cut around 3/4 of the way through then bend the pipe on the other 1/4. Then weld some steel over the openings. Won't be pretty (not with my welding anyway) but should get the bend I need. 

Just FYI, I think it was FMF that had a "secret weapon" in the mid 70's...  it was actually a sleeve that made the exhaust port smaller and extended into the head pipe a few inches.  Dyno testing showed power gains everywhere...  even at high rpm.  Bigger is not always better. 

Making some progress. 
The holes on the RZ exhaust flange did not quite line up with the DT studs.
Drilled the holes out from 8 to 9.5mm. Sorted that out. 
Made 2 cuts in the exhaust header and some small bends achieved the required angle.
I am planning to cut the existing mount of the exhaust and make a new one that will attach to the foot peg shaft.

Nice work! 

Removed the original rear mount and added the new one.
don't judge my welding please!
Next to weld up the cuts in the header.