i have seen those 3/4" adapters in cast, as well as "billet". also, i "think" i remember seeing a 1/2 or 5/8" cast [kinda chintzy] adapter that needed a little "tuneup" to clear the secondary throttle blades.
another thing you may consider, would be to space your K member from the front rails like the 68 hemi darts. those spacers were what, 3/4 or 1" ? that would give you extra room under your hood for sure, and would not be hard to fab up those spacers if a guy knew what the dimensions were.