Blocker rings are the answer for missing gears. Here is an excerpt that explains and a link to the entire article:
According to Hanlon, the blocker rings are essentially the same basic design from the '60s-era Top Loader transmissions. The Hanlon Motorsport friction-lined rings were designed out of necessity. "As the horsepower and torque has increased dramatically, the transmissions need to be stronger and include heavier gears to withstand the power. The heavier gears have more inertia, so it's harder to slow down those bigger and heavier gears. The lined rings slow the gears down quicker to allow them to engage easier."
Read more:
Hanlon Motorsports Tremec TKO-600 Transmission - Muscle Mustangs & Fast Fords
I just bought a TKO 600 from Bob Hanlon (he's in Elverson, PA just NW of Philly). Nice guy to deal with and obviously knows his stuff. I have his phone number and address if anyone is interested. He said he and Liberty get the blocker rings from the same source, so they are the same - just a matter of where you want to get them.
Regards,
Kevin