Micro Seiki BL-51 and Which Tone Arm?

You won't have any problem mounting an MA 505 on a BL-51.
The manuals at Vinyl Engine shed light on the situation:

Micro MA-707 pivot to spindle: 222 mm (effective length - overhang)
Micro MA-505 pivot to spindle: 222 mm
Infinity Black Widow pivot to spindle: 222 mm
Grace G-704, 707, 714 pivot to spindle: 222 mm