Th350

The 640520 is supposed to work on most GM transmission including the TH350.
The casting port on the TH350 is a female 1/4"-18 NPSM. Other than factory OEM and a few after market companies this fittings is not used anywhere else in industry. GM uses an adaptor which threads into the casting port and seals with a washer and converts the thread to inverted flare for hard line usage.
If you want to use -6 AN fittings on the ends of your hose you will need a special adaptor to convert it.
640520 is that special adapter
Looks like maybe someone "fixed" it previously by tapping the straight thread to taper(or just forced the tapered into the straight thread hole, seen that before) in order to use common hardware store NPT fittings.
Have you tried a stock GM 1/4" NPS to inverted flare fitting to see if it will screw in? If it does Russell makes an inverted flare to -6AN adapter.
You could run a 1/4" NPSM tap in there.

The reason GM used a straight pipe thread and washer was to eliminate possible cracking. But in it's past life your trans may have fallen victim to some yeah who who stuck a tapered fitting in and fugged up the treads. Run a straight tap thru it and use a fitting with a sealing washer just like GM did and you won't have to worry about cracking unless you go gorilla on the fitting.

Putting grease on the tap will remove most of them. I have a set of bottle type brushes for cleaning inside of things. If you use a small one, way smaller than the threads and work it around the threads in a counter clockwise rotation (unscrewing) it should get any remaining. You aren't going in that deep. You are just restoring what threads were there, so there won't be a lot of shavings.

The threads for straight and tapered are 18 per inch. If the treads are clean either will thread in. Straight will go in until the sealing washer seats. Tapered will go in until it seals or the case cracks.
Chrysler started using tapered fittings on some of their 42r thru 48r late model transmissions. Sonnax now has a kit out for repairing the cracked cases.