anyone know if the 350z diff is based from the h190 or is from the bigger models?

Its just that 99 usd for a vlsd is crazy and so are the others online in the USA.
http://cgi.ebay.com/ebaymotors/NEW-OE ... uckQ5fPartsQ5fAccessories

Is it a viscous LSD?

It probably is not a Hitachi diff, they use an IRS diff that looks (at least superficially) similar to the 510 or 240Z unit (Fuji).

yep its a viscous
Not sure about make but i heard somewhere its possible to fit the r160 to the h165
and the r200 to the h190???

The Z32 300zx's used the R230 differential. Whereas the R32/R33/A31/C33/C34/S13/S14 etc use the R200.
There is a solid rear axle diff while was fitted to some navaras which was called the C200. I read somewhere that they used the same gizzards as the R200. I actually bought one of these diffs off trademe early last year. Side by side it looked VERY like an R200. I pulled the backing plate off it, and it was a CLSD too. The bloke I bought it off had bought it site unseen hoping it was an H190. But he than found it was too different to fit his housing. I would up on selling it to a mate of a mate for a CA18det powered KP60.
I would doubt that an R230 center (Assuming those 350Z'd still use that diff) could be used in a hitachi diff.