Hi ENO,
When you wrote the 73 frame was the same as the sidepipes(969&980)model # not the year,did you mean that they are SIMILAR, they are not the same,the sidepipes have their own swing arm and their frames "bifurcate"under the motor , the rear engine mount wont fit the later 400 or 250. the steering head stem is also 1inch shorter ,check the distance between the fork clamps.
This misconception is carried forward by an article in VMX ,sorry Ray,of an article about a so called "Factory Works Replica "CZ ,Vmx #4, not a 67 CZ , just a Yellow Tanker with a Sidepipe Tank .
If your are buying a Sidepipe be carefull, there is at at least one"cheater' out there
Real Sidepipes are becoming important in the VMX market place ,so check check check before you buy
Ps no I'm not an expert but I have built a couple and one under construction,