Does 'the' 64-pin connector (174518-7) that is being used for boomslangs fit the '99 ecu or is it only for 90-97 ecu's? I can't seem to find a lot of information on the '99 ecu connector.

 

When building my harness, I had the most trouble finding that 64-pin connector (although for a '97 Miata). No one had it in stock without a substantial minimum order. So, went to Tyco's webpage and found the part. Over on the right side is an option to request a sample. I tried it and they sent me the parts for free directly from Japan. I didn't even have to pay for postage. I didn't even put in fake info either--when the form asked for the project name, I put in "megasquirt harness". I did end up buying the rest of the multi-lock connectors and pins, so it was fair.
I know this doesn't answer your question exactly, but finding the connector is going to be your next step.
Good luck,
Chris