Hello All,
where is resistance wire located in 68 400s and what´s the easiest way to find it?
I have some trouble when revving and when I measured I got out only 5volts or so.
My guess is that previous owner who changed the HEI didn´t take out the resistance wire. At least this is the starting point. The carb and ignition timing are ok.

first get rid oh HEI !!!

I'm not 100% sure but I think the whole wire from the firewall junction block to the coil is a resistance wire.
You can check by taking a voltage test at the terminal block on the interior side, it should be 12 volts.
You can replace the wire , it's in the terminal marked K on the junction block. Or cut the wire near the junction block and splice in a new wire. I THINK the resistance wire has a certain amount of ohms of resistance per inch or foot if you left a small amount of the resistance wire and spliced in normal wire you may get close to 12 volts. Better to take the junction block apart and replace the wire though.
I mounted a relay on the firewall near the HEI controlled [turned off and on] by the by the original wire and powered with full alternator voltage [14 volts] from insulated terminal block also mounted on the firewall. If you want a clean original look change the wire.

Here are a couple links I found with some info that may be helpful to you.
http://www.pontiacstreetperformance.com/psp/HEIconversion.html
http://www.oldengine.org/unfaq/hei.htm
Please post some pics with what you decide when completed.
Gordy

Gordy, I noticed that I never answered to your post.
Sorry.
Thanks for the links. I ended up to rewire whole car with American Autowire Classic Update Series Wiring Harness Kit. I also rebuilt HEI installing new cap, rotor, module, 50000V coil, springs... No more problem.
Engine revs without hesitation to red line.

Another happy ending, good work.

Al yea, it's amazing how much the new wiring did to overall feeling of the car. When no more leaks from old loose connectors, bad wires etc everything works better.
Everything

Excellent!