I was thinking 39w ballast, but yes it's M130/C130 little electronic ballast. I realized also that I won't be able to monitor the lamp current since it's a HF electronic ballast.

Haven't tried traditional magnetic fluorescent ballasts on SOX (much), which I'm thinking would be more controllable lamp-current wise. So just to make sure, how do I wire in a starter if the SOX won't start right away? I have quite a number of F40T12-F30T12 RS ballasts which are low power factor, (one has a March 1961 stamp).

Is 0.6a enough to overheat the F40 ballast?

Yes, 0.6A i na thing designed for 0.43A is a huge overheat.
The power dissipation goes with square of the current, so 0.6A instead of the designed 0.43A means about double power dissipation.
Double power dissipation means about double temperature rise (twice the temperature difference between the internal hot spot vs the ambient temperature).
Normally the ballasts are designed to operate with temperature rise of about 70degC.
Doubling that means 70degC hotter operation than designed for.
And for each 10degC hotter operation the lifetime about halves, so 70degC hotter operation means 128x shorter lifetime than designed for.
So from a modest 100khours (quite common rated lifetime for the gear) you get lifetime about 800 hors (barely a single incandescent).
And this assumes the operation temperature does not approach any hard threshold like melting or chemical decomposition of some component or so.
When it does, the death becomes rather instant.

The preheat ballasts are intentionally designed to boost their current when the load is effectively shorted (lamp circuit in the preheat phase), to provide good warmup of the electrodes for start, but it is designed to spend just a few seconds in this mode at a time, so the thermal mass is able to swallow the extra dissipation. But operating it there permanently does mean a severe overheat.

I would imagine a 39 watt M130 MH ballast using a SOX ignitor would be a better solution?

I would imagine a 39 watt M130 MH ballast using a SOX ignitor would be a better solution?

But tge ballast has to be the magnetic HX.

I would not use electronic, because there is hard to predict their exact characteristic, mainly for these low wattage MHs.
The tging is, some electronic tend to have features designed to stabilize the arc tube temperature by actually varying the current. But to sense the temperature, they actually measure the arc voltage and ASSUME it follows the saturated vapor high pressure MH curves. So using a low pressure lamp there would get the ballast controller confused so it will in the best case shuts down for a "lamp error". Worst case severely overload all the circuits.

However many electronic ballasts could still be OK for tye 35W lamp, provided they just plain maintain the power. Usually they are afeeding a square wave current, with voltage limit around 400V, which should be sufficient for the SOX. But you should be sure the ballast is of the simpler constant power type and not the "extra constant color" type.

I am mainly trying to go for the magnetic MH ballast since I have a preference for magnetic ballasts over electronic ballasts.