If you take 4" out of the beam, the outer of the 5 spokes will be about where the std rims track was.
The spindles add 1/2" each side, and the wide 5 kits (CB ones) add some more to the track - about another 1/2 " each side I think from memory.
I believe Vintage in sydney have a kit which has 0 added offset, but im not sure if that is with drop spindles or not.. have to check with them.
If you want a really narrow look, you will have to take at least 6" from the beam. If you are narrowing to compensate for all the changed componenets in the front end, then 4" will be fine.
Kim's 60 has all the same stuff up front, with 90mm out of the front beam... check some pix here. It steers and handles awesome.
http://forums.aussieveedubbers.com/viewtopic.php?tid=27343&page=12