the oldest you'll want to go if you want to keep the stock engine bits is '74.5. The stock carburetion on a '75 is a single Zenith-Stromberg carburetor. '76 is the last year that they used your dash in the car, so that's the newest you'll want to go if you want to keep the stock dash.
If you want to change the carburetion and intake/exhaust manifolds on the car, you should be able to go all the way to 1972. I believe all the light lenses are compatible all the way to 1969(?) for a stock appearance. However, I think the interior of the car changed a couple times after 1969 before you car. So, maybe 1973 for interior bits.
hmmmm... Well, looks like 1974.5 to 1976 if you want everything to remain stock. But you can add in 1973 if you want to change out the carburetion system for twin SUs.
HTH