When I have to repair weld castings I boil them in a fifteen gallon army pot. I use a propane brush burner torch for the heat source. Keep boiling and replacing/topping off with fresh water. When no oil continues to rise to the surface it is clean. I then scrub with lacquer thinner, then 50/50 lacquer thinner and acetone. Then finally wiping with acetone. Of course there is filing for groove preparations. Hope this helps.
If you want the repair to last, 5356 or other 5000 alloy is required.
4000 series fillers tend to crack, due to the metallurgical incompatability.
Most common castings are magnesium alloyed. 4000 series fillers are silicon alloyed. The mag-silicide produced in a 5000-4000 weld can easily cause underbead cracking or lack of fusion defects.
Tungsten type, gas flow, etc have nothing to do with this.
Preheat, postheat and the like have little to do with this as well, as long as your machine is big enough for the job. A Syncro 250 should be plemty big for the work you anticipate.
As a cleaning technique, you may consider running your tig torch across the weld area to pre clean, as long as you are on AC current.
Try these suggestions. If you are still having problems, email directly.
Good Luck
brande