An interesting feature of steel frames is demonstrated by Surly frames: their rear end spacing is currently 145mm, which is neither 142 nor 148 (the standards).

An interesting feature of steel frames is demonstrated by Surly frames: their rear end spacing is currently 145mm, which is neither 142 nor 148 (the standards). What they say is that because it is steel, you can bend it a couple of millimeters without any negative side-effects, the frame will tolerate it and will not break. So your frame can accommodate both sizes.