So, is a preamp needed in such an application and, if so, what kind of preamp?
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NO. You should not require a pre amp for the SM58 and your transceiver. A pre amp is likely to overload the input state on the transceiver - inline pre amps typically provide between 20 and 30 dB gain, which is not required for this application.
The SM58 has a medium-hot output level (open signal voltage on the product data sheet) and it should drive the rig well enough. Make sure you avoid - do not use - the 5v bias voltage intended to power FET type computer gaming headset microphones and the like. You will likely have an unbalanced connection, so the bias voltage does not become or act or even seem like phantom voltage which only works as a phantom on a balanced cable/input. The FT900 has a front panel 8-pin modular, RJ45
network type connector - you need to be careful how you wire that . I am not sure if that older model expects a condenser capsule or a dynamic cartridge. I believe it uses the standard Yaesu Hand Microphones MH-31A8J hand mic, which has a dynamic cartridge. Your Yaesu user manual should have pin out charts for all connectors, which will guide you in building a suitable cable / connector. Otherwise, vendors on eBay.com and elsewhere sell suitable cable adapters to mate the XLR on the SM58 with the 8-pin modular, RJ45 connector on the rig. You will also have to pay attention to the PTT connections, as the microphone on-off switch (if present) will only disconnect the mic from the rig - it will not activate the transmitter as a PTT switch - at least not without substantial re-wiring! K8JHR
