Buy them from jegs, cheapest mag replacements... I think they are the same as the speed masters. Start cheaper then do the work I show in the speed master magnum inspection thread and you'll have yourself a good 260 cfm head.
Guides to 11/32, cheaper +.100 1.94/1.60 @5.011 length valves, cut for std springs w/1.44 cutter.. then a good valve job with chamber sweep / deshroud cut and hand blend. There are no bolt on magnum heads worth a **** atm. That's the only path available that I'm familiar with.