Aloha John,
For sure, by all means!
It is entirely up to you how to use any free space you may have below your
cabin sole.
I'm certainly no expert, but...
For the sake of stability... I would suggest you use the space for something heavy such as fuel / water tankage or
batteries.
If you decide to add another tank athwart the
keel then it would be a good idea to add baffels in order to reduce Free Surface Effect.
If the space in question is fore & aft on the centerline then I'd put
batteries there.
I reckon it primarily depends on what your future intentions with your boat.
If you plan to meander along with the
trade winds then water may be best.
If you're crossing the equator then fuel may be better.
If you're just gonna hang around
Hawaii then I'd fill the space with da kine.
It's up to you.
It sounds like you're in a very good position to create a custom
interior layout just the way you'd like it... which is enviable. Just make sure the berth and settees are at least as long as you are tall.
But - to answer your question... I've never really wished that my fuel tanks were
bigger.
I departed the Ala Wei bound for Saipan in 1995 with 40 gallons of fuel and arrived 31 days & 3500 miles later with 36 gallons... only because my
refrigerator was broken. The most fuel I
ever carried in the 25,000 miles since then was about 100 gallons (aboard a 37 ft boat powered with a 30 hp Yanmar) when we set out from the
Canaries across to the
Caribbean. And this time the fridge
was working... and we still arrived with plenty fuel.
I'll probably do it differently next time... but I'm really not sure.
But that's just me.
Carry on.
Kirk