That wasn't the final Indy motor then. Navarro's site states "twin turbo-charged Rambler six producing 700 hp". I think the 700 might be an exagerration, or liberal rounding up, but the twin turbos should be accurate. The FIRST Indy motor only produced 550 hp though, so you're on track if that is indeed the first engine. The FINAL engine supposedly had the twin turbos (likely in series -- one blowing into the other -- so it's possible you only saw one?) and produced 640+ hp.
OK, I was wrong being wrong. There are two turbos clearly (sic) visible in this bad photo:
There's two waste gates, so I guess the exhausts are in parallel? The compressor side must be in series.