I've not found any local shops that have converter flushing equipment. The Ron Sessions th350 book shows such a machine, but no luck finding one.
With my race converters, I have to have them cut apart for cleaning and inspection. This would be the same for a stock or modified converter. You can only dump so much out through the pump drive collar, and some will remain.
For a stock conveter, it would be easiest to go to any trans shop and simply get a rebuilt converter which should be pretty cheap.
Another option is to put the converter in a parts washer and spend some time running solvent through the converter to flush what you can, but it won't be 100% clean and if there's any wear, you won't know it because the only way to tell is to cut it apart.
79 Malibu, 414 ci BBC, 3550 lbs.
10.75 @ 125 mph, 1.49 60'