Hello Scrooster,
Your project sounds quite do-able. Main limitations will be how much noise you can tolerate in your location. A 3600 rpm single will be loud! Economy will improve at reduced revs, but power output also reduces. You quote about 8hp at max revs, so at 2800 to 3000 you should still have near 6hp.
A handy rule of thumb says you will get about 1kw for 2hp, so reckon 3kw max at 3000 rpm. A gen head several kw larger is ok, as the losses are not much greater, and starting heavy loads will be a little better. Assume your gen is 60 hz, (1800 rpm) so your pulley ratio will be 15 to 9 reduction, say 7.5" to 4.5" pulleys, or 8 to 5 would be ok. Twin A belts or poly vee in these sizes would be fine.
If your need is for standby or portable power, mount on a solid frame and pin it on top of an old truck tyre, but for continuous run, vibration will loosen everything and cause you much pain, so bolt and grout it to at least half a ton of concrete. Buy earplugs if you have to sleep near it. Good luck, Regards, Combustor.