Fred, I'm assuming you're wanting this for standby or emergency power, so you'll want a synchronous alternator and not something you'll be running in parallel with the local utility? So you'll need to decide if you're wanting to run the engine at 1200, 1800 or 3600 RPM which are the synchronous 60Hz speeds for the US, or 1000, 1500, or 3000 which are the synchronous 50Hz speeds for the UK and Europe.
All of these speeds are well below where most V6's develop their best power, so you'll have to check the power curves on these engines to pick the best fit. I would also derate the manufacturer's HP or kW ratings by at least 25 if not 50% to get better longevity from the engine, since it'll be seeing continuous duty as/when you do need it.
Lastly, are you planning on using LPG injectors or converting the engine over to an LPG carb setup?
Keep us posted, this is an interesting application!