I don't know if this will work but it seems like it might, if my limited understanding of how the tach works is close.  It uses the GM HEI module as the "pulse" source for the tachometer.  One scheme is for the 240Z current-reading tach, and the other is for the voltage-reading 280Z unit.

Serendipitously, the factory GM HEI module limits current to about 4 -5 amps, apparently.  The current through the 240Z tachometer should be about 4 amps, so it's a pretty close match.  13 / (1.6 + 1.6).  Primary circuit coil resistance and ballast resistor are both 1.6 ohms.  13 volts is nominal battery voltage.  The 240Z tach just sees voltage rising and falling as the coil charges and discharges.  The HEI module charges and discharges the 240Z tach, in essence.  The current limiting of the HEI module should protect it from too much current, although I'm not sure if it responds fast enough.  Might need a resistor in front to help out.  The primary resistance of an HEI coil is about 0.5 ohms.

The 280Z just sees voltage rising and falling and has a big resistor in front of it.  So all that should be needed there is the connection to the C terminal to see the voltage change.

Here are the schematics.  I'm no electrical engineer and could be way off.  I saw the wiring schemes for the MDS 8920 adapter though and this thought came to me.  Please pick away at it, if you know something or have a suggestion.  Or just blow it up with massive electronics knowledge.  I can take it.  My main concern is that the HEI module will burn itself up with no load on the C circuit, but the current-limiting should cover that, I'd think.  Or the 240Z tachometer will fry, but again, 5 amps should be handleable.  Any experiments could be done with a 5 amp fuse in-line to save parts.

One other benefit is the automatic shutdown of the HEI module if there's no activity on the C circuit within a certain time.

Just trying to keep the old brain sharpened.

240Z

G - ground
W - white trigger wire
Mount grommet - ground
C - ground
B - tachometer - battery power
HEI module passes ~4 amps, current-limited.  
240Z stock has 1.6 ohm primary + 1.6 ohm ballast resistor, 3.2 total.  13 volts (nominal) / 3.2 = 4 amps

 

280Z

G - ground
W - white trigger wire
Mount grommet - ground
C - factory resistor - tachometer
B - battery
280Z tach only sees voltage pulses, essentially no current.  The module just lets the voltage pulse from ~0 to 13 as the trigger switches the module.  It's a "phantom" coil.

I don't see any schematics or links to them. Anyhow, if you are assuming you can short the output of the HEI module and rely on the internal current limiting to protect it: I doubt that will be reliable. You will need a 3 ohm (or so) resistor of suitable wattage as a load. The waveform that results from the points and condesner is rather messy and has a lof of over-shoot and ringing that the tach may have been designed around. I repaired my tach a while back and could not get it to respond to a squarewave generator on the test bench. Considering the foregoing, I wonder it the 4-wire tach will respond to the reasonably clean rectangle waveform that is likely the result of your suggestion.

26 minutes ago, beermanpete said:

I repaired my tach a while back and could not get it to respond to a squarewave generator on the test bench. Considering the foregoing, I wonder it the 4-wire tach will respond to the reasonably clean rectangle waveform that is likely the result of your suggestion.

That's interesting.  I don't claim to know exactly what the tach's use, I just tried to give them what they're probably getting, with either points, or electronic ignition.

I actually mentioned the possible need for a resistor, thanks for the second opinion.  I've read of people burning up their HEI modules by using a later model ~0 ohm coil that the computer controlled HEI systems used.  They look the same but they're not.  But that was just one guy.  If I try it I'll start with a resistor or potentiometer and work down.  Stop when it works.

I went too liberal on the use of the word schematic.  It's a word schematic.  Anyway, if I ever find one of these old ignition boxes for cheap I'll probably try it out.  I just wanted to throw the idea out there while it was still in my head.

Just FINALLY got my MSD 8920 tach adapter to work on my 71 tach, which we know is current type tach. I run a Megasquirt system which has a tach output just like a MSD ignition box. I tried for the longest time to get it to work with little results. I was trying to pick up the tach -white loop wires further down the harness under the dash. I swore I found them using a ohm meter for continuity , but still no tach. It would actually just barely move the needle. Today I got desperate and pulled the tach and wired the tach adapter directly at the plug-now it works perfect. The black/white and green/white come off the tach loop, but must get sidetracked thru the ignition or fuse box , so I bypassed everything by splicing in right there. Though my wiring for ignition is a bit different , this should work for anyone that is running a separate box for there ignition that has an tach output wire. 

Could the original wiring still have been running through the 240Z ballast resistor?  Maybe you just had too much resistance. Continuity would still be there.

No ballast. I'm running 6 LS1 coils for ignition. I know just enough to be dangerous about wiring- but it works!!!

Can you post a picture of your coil mounting?  I'm mulling over various ideas for coil setups.

http://forums.hybridz.org/topic/113728-ms3x-install/page-2