Mountain bike frames and 42t chainrings don't go together. The short chainstays and wide tire clearance requirements of modern MTB frames, doesn't leave enough room for chainrings that large. I have built(welded together) a number of MTB frames and its always a challenge fitting the chainstay between the tire and the chainring. On the plus tire (3" tire clearance) frames that I build, a 32t chainring is the max. that will fit.
By the way, quality sliding or rocker dropouts cost me around $150, which is around the same cost as the rest of the frame tubes, head tube and bottom bracket. It would cost me over $300 in raw materials to build that frame and that doesn't include powder coating it or me making any money from it.