1.5, 1, 1.5, 7, 1
This of course works well, as do 1.25+1.25+1.25+6.25+1.00+1.00+1.00+1.00+1.00 without altering the symmetry of the spacebar location, or using any funky size keys. It also allows "vim style" arrow keys, with a regular shift row.
(Look at all those 1.00X keys!)
I'd actually be a fan of 1.5 - 1.5 - 6 - 1.5 - 1.5 - 1 - 1 - 1 (kmiller8's option is good as well).
6 unit spacebars are a little harder to source but there's lots of cherry boards out there that came with them.
in the end though, it really throws off the symmetry of the board as a whole so it's a hard decision for me.
I think that as long as it is a size SP supplies there is no big problem. In the end there would need to be keys with custom printing made anyway (doesn't really apply to spacebar of course..). And the shift keys are also non-standard widths.
My best effort keeping as many of the modifiers as possible is to use a 4.50 spacebar. I don't know if this is Japanese standard in general, but Filco uses them on their boards, and they are also available from SP. I also tend to use only my right thumb for space myself, but that could probably be trained away =)