The bracket you describe is for a Gen V BBC that never had a manual so there is no pilot hole in the crank for a manual input shaft, nor is there a drilled and tapped hole in the block for the clutch linkage cross-shaft. That bracket fixes the missing hole issue.
Your Mark IV BBC (two piece main rear seal) 454 block should have two holee drilled in the block; one for a Corvette, and the other for your car. The clutch should bolt up with the short ball and the other end is welded to the frame. You will need a 349-409 cross-shaft as they are shorter to fit the wider block into the frame.
Exhaust manifolds may not fit in your car's frame as it never had a Mark IV installed (the original 348 was a Mark I BBC), because the frame narrows at the rear. There may not be enough room for clutch linkage and stock cast iron exhaust manifolds. ZL-1 cast iron exhaust manifolds off of a NASCAR engine from 1962-'63 will fit as the 409 had Mark II canted valve heads similar to what is used on your Mark IV ((segregated exhaust ports, unlike the Siamesed "W" engine lay out). But they are as rare as hens teeth today.