Is there a difference as to year on the rear valance, would a 75 fit a 74?

I believe 74 and 75 were the same.

Yes. The snow plow was made from 70-72. The short valance came in 73 and will fit 73-76 I believe.

how much longer is the early valance? Was it shortened to aid in cooling?

.....It works!....lot of customs out there also!

The early ones packed too much debris between them and the muffler. Also cooling is better with the shorter valance.

The pre '73 valence is about half the length of the early valence ( approx 3.5" versus 6.75"). The later valence is, in my opinion optimal if you don't care about originality, and both fit all model years with modification of the early valence for exhaust outlet.

If you have a '73 & later 914, the shorter valence is proper for all subsequent years. They are still readily available but most will require rerfinishing.

To find a usable early valence is the Holy Grail. They're out there, but expect to pay big bucks before the cost of proper refinishing. Ask ne how I know!