I have one of the amps but I bought it from another ham, don’t know if he had to assemble the keying cable. 8 pin din plugs are a bit fiddly, but you can always buy an assembled cable and adapt it with a pair of side cutters.
I experimented with the ft817 driving the amp. According to my power meter, a cross needle meter included in an ATU, the amp produces just over 40w whether it’s driven by 5w or 2.5w. This means 5w is overdriving it and will produce a wide signal on ssb. There is no ALC feedback connection to reduce the drive, and anyway those systems still produce dirty signals.
At 1w input it produced 20w and at half a watt it was about 10w. So it is apparently linear-ish at low power levels, typical of solid state amplifiers, but output flattens off with higher drive. 2.5w is probably the practical and socially acceptable drive limit for good operation.
Your amp may vary, but I recommend testing this aspect.
A fringe benefit of this outcome is that the 817 can run at a lower power level, resulting in a modest saving in battery consumption.
73 Andrew VK1DA/VK2UH