Hi digby,
it's been quite a while since you've posted your inquiry, yet nobody seems to have replied to date... so I take the liberty of trying to do so (although you may have received all the info from someone or somewhere off-forum in the meantime).
As far as the
string lengh is concerned, just go for (regular) long scale strings and you should be fine.
Although the actual scale(!) lengh on a 4003 bass is a bit shorter than -say- on a F****r bass, there is considerable space between the bridge saddles and the "tail end" of the tailpiece (where the strings are anchored).
Hence the need for long scale strings, albeit a not-quite-longscale instrument...
If you're not sure which
string gauge to pick, I'd recommend comparing the string tension(!) of the La Bellas with your existing set that's on the bass right now, and get the set that matches most (if you're happy with the current action and playability, that is).
This way you should need only minor (re-)adjustments on your instrument, and it might make it easier to adapt your playing style to the new flatwounds.
Hope that helps...

Cheers,
kiwi