Those larger "drop outs" will provide a lot more support where obviously there is a design flaw. Perhaps not a "flaw" per se, because none of these machines were ever intended to support upwards of 230 pounds.
Also, using a slug "might" require bending the frame more than it wants to bend WITH those "drop outs" in place. Because I like to weld, and have plenty of time to do this stuff, I'd consider cutting off both of the "drop outs," "then" cutting the cracked area of the frame completely in half, and then separating the two pieces and installing my solid slug. Holes would need to be drilled first on either end to "capture the slug, as well as to gain access to push it back up into the mating section once the two pieces were aligned. Yes, this is probably too verbose and too much information on a social networking platform, but the way I just described is the only way to do this right. Running a weld bead on the crack is not going to mitigate the fact that the design is prone to cracking in the first place.