Perhaps the early corvettes used a distributor tach drive with the 6 cylinder?
If you just want a tach and don't have to have a mechanical one try
http://www.tinytach.com/tinytach/index.php I had one in my truck for a while, very easy to install and mount, small too. Worked great for my needs.
If you want a nice electric pickup tach with about 3,500 max try Stewart warner, I will be buying one from them for my diesel truck.
Grigg
