I'm running a TKO600, and I bought it with a Tremec bellhousing. It's my understanding that the 3550 and TKO were replaced by the TKO-500/600 product line back in 2004, and that today the 3550 and TKO are only available as used or rebuilt units. I'd say that you have the wrong bellhousing, and I'd talk to a Tremec rep, like Mike Forte of Forte's Performance, or one in your region.