The solid metallic bushing options available for those two locations are not legal. If you found a non-metallic delrin/poly version of those two bushings they would be legal. I have seen some delrin options around, but am uncertain as to the benefit of changing the bushing in those two locations. They seem to serve more as a vibration dampener rather and the benefit to going solid or delrin/poly wouldn't really be the same as the other locations that serve to maintain suspension geometry throughout the movement range.
Directly quoted from the rules so newcomers to this thread understand why:
15.8 Suspension Bushings
Stock rubber suspension bushings may be replaced with any non-metallic bushing. Stock bushings, consisting of rubber and metal, may be replaced with a combination of non-metallic/metallic bushing so long as the metallic portion does not exceed that of the stock bushing and the geometric relationship of non-metallic/metallic is maintained. Factory 968 style caster blocks are allowed. No bushing may alter original suspension geometry.