A brief glance at the local ads shows I can pick up a BB 727 core (or 2) for about $200 +/-. Are there better years or versions to look for, if there's a choice?

1970 & earlier:
- narrow bushing high clutch drum with 2 oil ports 180° apart.
- Flat snap ring used in high clutch drum
- corresponding narrow bushing journal on front pump reaction shaft support and large oil port for high clutch piston.
- Solid kickdown servo piston with 1 or 2 springs
- Different piston lip seal dimension wide vs. short lip
- 1966 & earlier had 19 spline input shaft
- 1966 & earlier had 10 spring peg high clutch piston.

1971+:

- Wide bushing high clutch drum with 1 slotted oil port.
- corresponding wider bushing journal on pump reaction shaft support and smaller oil port for high clutch application.
- Controlled load kickdown servo piston with oil cushioned inner piston with only 1 spring (shim solid or plug the side hole in the piston)
- Flex bands introduced
- Wavy snap ring in high clutch drum
- 1967+ had 24 spline input shaft
- 1976+ had 37.5° pressure angle on front planetary for improved strength
- Part throttle kickdown valve added
- Lock-up converters 1978+
- Smaller orifices in VB to feed servos with less harsh shifts
- 15 spring peg high clutch piston

Many more differences......
My notes show the planetary spline pressure angles were:

'62 to '75: 45º
'76 to '93: 37.5º
'94 and later: 30º