The problem with mating a 727 to any '61 and earlier V8 is the crank flange, the early flange extends further from the block, has 8 bolts and registers the flywheel/flexplate differently. Most Hemi-to-727 adapters are fairly thick in order to space the trans back and correct the bolt pattern.

In the pic below the crank in the background is the earlier with the extended flange.