I'm not sure if the power is positively the current problem, but hot glue would probably end up a problem in the future. Buy a female Molex connector from Rat Shack, if that doesn't fit, just use the inside connectors by themselves.
If your scan converter has composite out, using that should be fine. Are you using a J-Pac to amplify the signal? (that might help if you aren't using one)
Also, don't run your Dreamcast through the scan converter, disabling a pin in the VGA box will make it output the 15kHz signal needed for arcade monitor use.
http://www.gamesx.com has the pinout for the Dreamcast A/V connector, use it to determine the pin to put it in VGA mode, trace that wire into the VGA box, and cut it.