....... or, if your 55.2-59 GMC has a factory V8 like mine did you can do it this way.
Get a Sun Supertach II. I found mine at a swap meet for $15 (in 1992).
Remove the chrome bezel from the front of the tach and use a dremmel tool, hand held sander, etc to remove about 1/16" of the OD of the open end of the tach case where you removed the bezel so the tach will slip into the stock GMC bezel. Buy a decal kit for GMC instruments. Use a sharp Xacto knife to cut out the numbers to reface the tach to 4000 rpm (you won't turn the stock GMC [Pontiac] V8 any faster than that). Get some paint from a hobby shop and paint the red tach needle to match the GMC needle color.
The trick is to switch the tach to 4 cylinder mode before installing. 8000 rpm (full scale) in the 4 cylinder mode is the same as 4000 rpm (new full scale) in 8 cylinder mode.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
