I am using a Nissan R200 lsd in my build.. ratios are available from 3.5 to 4.8 and viscous LSD centers are very cheap. Problem is, it is very heavy.. about 40kgs.
I am about to replace it with a subaru R160 lsd. This weighs 22kgs. In the USA you should be able to find plenty of Subaru diffs in a wide range of ratios. Hope this helps.
http://lotus7.co.nz/forum/viewtopic.php?f=16&t=72