It depends on what kind of riding you do. Tight trails with small spaces in between trees and stuff, spacers are bad. If you have wide open spaces that are fairly flat and you want to slide, spacers are good. If you have a real rocky or pockmarked area you ride in, beware. Spaced out rear axles can bite you if you are not careful. Narrow rears are easier to slide than wide rears, that's why racers use wide and offset rear axles, to help prevent sliding and get bite and leverage.
Nicholson 500x (Project)
Nicholson 185s
Nicholson 110 (That takes an atc70 tank)
Ascott 500 head/350x (Project)
Homebuilt Racer Chromoly Suspension 110 (Ragin Runt)
PK Racing Suspension 70
A+ Inc Suspension 70
AWS Aluminum 90 frame
Hi Performance ATC Suspension frame rd350
Hi Performance ATC Suspension frame 90