I figured it out.
I asked a friend of mine about a divide by 10 circuit, and he suggested that a gas tach already has the circuitry, all I need to do is either adjust it or modify it for my needs.
So I opened up the tach and sure enough there are three pots (aka potentiometers, variable resistors) . One for each position on the tach, 4 cyl, 6 cyl, and 8 cyl) so I recorded the value of the 8cyl pot, 179 ohms I think the other two were progressively lower, so I knew I needed a larger pot.
I went to my local electronics shop and bought a three leg single turn 5 k ohm pot. I bent the center leg over to one end, it doesn't really matter which and soldered them together.
Then I removed the existing 400 ohm pot and replaced it with my 5k pot.
Out to the truck to give it a go. I connected the signal lead of the tach to the W pin on the alternator, and it works like a champ. I adjusted the pot until the tach was idleing at 600 rpm, my guess as to what it is actually doing.
when I rev the engine it reacts as I think it should. I'm going to buy an optical tach so that I can calibrate it precisely.
I also left the glass off as I am going to replace the numbers with 0 to 4000 in 500 increments instead of 0 to 8000 in 1000 increments.
btw removing the glass was tricky, I used very small flat blad screwdrivers to pry off the bezel. It would be very easy to break the bezel.
Although I haven't been in to many tachs I assume that they are all pretty similar, it is really just a matter of measuring the pot for the 8cyl position, and then buying a pot that is ten times the value. Since 5 k pots are much more common than 4 k I just got the 5k. I suppose you could experiment with reisitors until you found the right value, but the pot makes it easy and calibratable. I even drilled a hole inthe housing so that I can calibrate it later with out disassembling it.
Any body with mediocre soldering skils should be alble to pull this off.
Oh and the White pot is the 5 k pot I installed. The metal pots are the original styles