http://www.howtoalmanac.com/kevin/pr...chfixtruck.htm
This is a walk through on repairing the tach, you can use this guide to convert a V8 tach to 6 cyl as well. C50 trucks also usually had 292 as well.
Check ebay too, sometimes you can find a deal. Sold a non-functioning tach dash on there a year ago for 50 bucks. Tach didn't respond, and I didn't want to bother fixing it since it was the 73-80 style.
If you do find a non-functioning tach, Tachman can restore and calibrate the tach for you as well.
http://www.tachman.com/oldsun.htm