When your checking your battery, are you putting it under a load? What kind of battery are you using? NICD, NIMH? I don't know off hand what the max amp load is from the gen 1 ignition, but I do know that I have the same engine, and had the same problems your describing, till I switched to a larger battery. It's especially telling when you say it happens at WOT. That's when the battery will see the largest amp load and if the battery is too small to support the load, the voltage will start dropping.

Please keep us updated as to your findings.