Can anyone tell me:
Will a 1980 PE400 tank and seat fit on a 1979 PE175? From the photos it appears the tank will fit but the seat would be to long.
The tank will fit. The seat will not. The C/N (both 175 and 250) has a steel base that is pretty much flat with an indent at the rear, whereas the later seats had a huge up-sweep that leaves them sitting miles about the tabs.
'Tex' on the PE mailing list has great experience in getting non-standard seats to fit his 175N. Ugly, but functional.
Early C/N tank had a "12" in a circle (on the underside) which signifies 12 litres. Slightly later versions had thicker walls (the plastic of the tank itself) and dropped down to 11.5 litres in capacity. Shape and mounting points of the tank remained the same. B models and UK had the early aluminium tank and different seat but that is not relevant to your needs.
You may wish to explore an RM seat and tank if they are more readily available to you. The 175 C/N is pretty much an RM125A frame.
Luke