This looks like a 330 µF, 6.3 V capacitor.
You'll be hard pressed to find an equivalent electrolytic capacitor. Right you are in noticing that tantalum capacitors are well known for being unreliable. However, you can improve reliability by using a 10 V type instead of the 6.3 V type - provided you get hold of one of the same size.
Electrolytic capacitors are not without problems of their own. They can (and they do) loose electrolyte over time. The warmer, the faster. As the capacitor in question is quenched between two coils which probabaly get warm, this is not the best place to mount ana electrolytic capacitor.
My suggestion:
- Get a 330 µF, 10 V polymer tantalum capacitor (the polymer electrode makes it less sensitive to mechanical stress).
- Solder carefully and quickly - heat will put additional stress on the capacitor.
- Get the converter up and running with no load or only a light load for starters. This will minimize ripple current in the capacitor, thus minimizing self-heating.
Give the capacitor at least 10 minutes to settle (this will improve the stability of the dielectric) before using the converter under full load.