Since you have a dialog with Evans I'd say that's your best bet; I realize now that their flasher circuit operates with AC or DC power but I'd only be speculating and having you perform shot-in-the-dark experiments whereas they should know right away how to troubleshoot. I don't think it's worth spending more time on capacitor values to additionally smooth the bell/flasher pulsed DC voltage until you hear from them.
What's important is to determine the operating voltage range of the coil to reliably open the GM door and slide out the man so you're measuring the DC voltage under load. If a PTC is installed, put it right on the wire that goes to the coil terminal on the GM frame.
I understand your combination of components only works when you tie the "+" outputs to the common. I've already opined about how to approach this so I'll leave it at that. If trial-and-error shows that it works to your satisfaction with different voltages from the two converters then so be it.
