You have the same problem at opposite ends if I'm understanding right....My 66 box had the large return fitting, now manual, my GTX box has the small.....
When the federal style pump went on my GTX I changed over to a saginaw style and the return hose is the small size so no problem..The pump I replaced had the small size also so maybe in the past 40 plus years the pumps have been swapped? You might be able to just unbolt the fitting from an old swap meet junk box and swap them or change pumps....I looked in my 67 factory manual and don't see any factory adapter type fitting.
My 66 had the same size return fitting pump and box and the 67 did also...