On the 70-73 the lower engine mount bolt went thru the frame, in 74 when they redesigned the frame the bolt went thru a mount on top of the frame.
On my 75, the same as the 74, there is a tab on the frame behind the lower engine bolt, it has two threaded holes. The headpipe has a tab that bolts to the front hole. The rear hole is for a p-clamp that goes around the nut that holds the muffler to the headpipe. Parts #7 and #14. In 5twins pic.
I think you can use the unknown headpipe by building brackets to fit as in 5twins pic.
On the later Specials the exhaust being welded together only has a tab that bolts to the front hole of the tab on the frame.
I haven't tried it but I think the tab on the frame will let you use any of the standard headpipes on any year frame.
The way the muffler hangers attach to the frames are different between the standards and Specials. So bit of bracket making will be needed there.