One way or another it want to be fitted without stress, so if the tabs on the frame and the tabs on the rail are both square to each other ,then machine a spacer to fill the gap, with an apropriate diameter, no good bolting it up and bending something, just a tiny thing that proably would do any harm , I just dont like bolts to be "bringin things together" they should sit nicely to each other and the bolt clamps the parts up tight.I have seen Laverda triple engine mounts (the gearbox end) with very thin shims to take up a discrepancy between the mating flanges, well why not, if the motor sits square by gravity,on the bottom fixings, and the chain line is good, then no sense is forcing something together with a bolt.
Not easy in many cases to get a couple of spacers measured and made, if access to the right machines and people is not close by, but well worth the effort.
CLEM