Well done on ordering new rectifiers. The ones that ship with these ST heads are garbage. A few years back I did a tear down and rebuild of a brand new ST head, I posted a lot of info and pictures showing how to do it. I replaced the rubbish bearings, skimmed the slip rings which were not concentric with the shaft, replaced the cheap carbon brushes, rewired the top box and fitted a new rectifier with an aluminum heatsink. Later I fitted an automatic voltage regulator.
Aluminum heat sinks can be found on ebay and other sites, old computers all have nice heatsinks on the processor chip. Drill a hole in the back of the top box, bolt the rectifier to the inside of the top box with the heatsink on the outside where it will get good airflow
The wiring is pretty simple there are two wires that can be traced back to the slip rings, these are the AC feeds to the rectifier, it does not matter which way you wire them as long as they both go to a rectifier lug labeled AC. The other two wires go to the field windings and are positive and negative, on mine the positive was the red wire. I do not know if your wiring has spade connectors or screw lugs, fortunately the rectifiers you ordered are compatible with both.
I think the fact that it generated AC for a short while with the replacement rectifier indicates that the windings are probably OK.
How do rectifiers fail? A bridge rectifier contains four diodes, the symptoms vary depending which of the diodes has failed and if it failed open or closed.
Best of luck, let us know how you go.
Bob