You need horizontal dropouts or track ends. Don't use a spring chain tensioner.
H dropouts are almost never used on any quality MTB frames. Single speed MTB is your best bet but check that they come with rack, mudguard and water bottle eyelets. You may find some low end ones with H dropouts.
H dropouts are more easily found on touring bikes.
If you are lucky you may find a 26" frame with eccentric bottom bracket such as Scott sub 10 or my own Dahon Cadenza. But old EBB can get corroded and stuck.