Won't Shift Into Gear While Running

I had this very problem when assembling my Barracuda after an engine rebuild. I had used a new crank and the hole for the pilot bushing was not drilled deep enough. The transmission slid right in and the bolts were tight with no gap between the transmission and the bellhousing. But the transmission was jammed in there just enough against the crank that releasing the clutch did nothing. If the engine was rebuilt with a new crank and everything else is the same, this might be your issue.