If you've got a turnkey Butler powerplant, then I'd say you should ask them for a converter (and rear gear) recommendation. They ought to know more about where it will be "happy" than anyone.
But, since you asked, my 461 has a 3200 rpm converter behind it. I wouldn't go any looser than that. At 3200 rpm in high gear, the point where the converter stops slipping, I'm at 70 mph with my gears and tires. Whatever you choose, you'll want it to NOT still be slipping when you're at highway cruise rpm, whatever that works out to be for your combination. Otherwise you run the risk of overheating the fluid and starting to cook transmission parts. Most of the heat that gets generated in a transmission comes from the converter.