Assuming you found the correct item as the flasher unit, just about any two prong flasher will perform as it needs to. The thing that concerns me is that there is sometimes a 7 volt regulator for the fuel and temp gauges that you need and I first thought you'd substituted that. Gauges still work? Is the behavior now any different from before changing the flasher?

On my 69 F100, when I have the emergency flashers running and step on the brake, the lights go solid, but that is the only time this happens.

Oh, and the same flasher runs signals and hazards.