The pre-'62 BB has a crank flange that's longer, I know of no adapter to mate the early trans to the later motor. You'd have to replace the later crankshaft with an early extended flange one.

As an alternative you could install a later BB to a '62-'65 cable-shifted 727 Torqueflite, the '62-'64 PB cable will mate to your existing PB shifter. Problem is, there is no provision for the Park mechanism in the later trans.

There is a '62 (one year only) BB 727 trans with a parking brake mounted on the tail like your '61, this is the easiest way to go but these transmissions are rare.