Necking down a 308 case will be a better bet. It will put less stress on the brass. You end up with slightly longer brass and slightly thicker necks. This is a good thing and allows you to outside turn the necks for a nice tight fit.
With lesser brass you take a chance of splitting the brass when expanding.
As for dies I prefer Hornady and RCBS. Hornady dies have the tear drooped shaped expander ball like mentioned by Robo.
Like Robo said anneal fired brass. This can be done a few ways. If you want to go big go get a brass-o-matic
and do it that way. Or do it the poor mans way.
1. Get 9x12" cake pan (Steel from wife)
2. Fill with water enough to cover bottom 1/3 of the case.
3. light up propane or MAPP torch and heat necks evenly and then knock them over into the water.
1. gather brass.
2. light up torch.
3. done mechanix ware gloves or something thin like that.
4. hold brass in finger and heat neck while rotating brass in fingers. If you burn your fingers you went too long.
5. drop in bucket of water.
NOTE: if using hand held method I found dipping fingers of gloves in water slightly will help when annealing 200+ cases.