Without more details, it is hard to answer your question. Why not just buy an early w114 if that is what you want? It will cost more to convert one to look like an early one, than it would cost to just buy an early one. The other reason your question is hard to answer is that you don't say what level of exactness you require. For instance, there are obvious differences like engine capacity, steering wheels, bumpers, brake lights, etc. But there are much more subtle differences: the overflow nipple on my 1968 is soldered/brazed to the radiator neck, whereas on a 1973, it is a plastic nipple simply pushed through a hole on the radiator neck. Is that an important difference? Not to me, and probably not to you, but who knows???

__________________
1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).