A 393 should with a decent set of heads should be able to do at least 600 on the motor.
I think we are getting a little bit carried away from what the guys is asking. We are telling him to build a 500 horse stroker motor to replace a 190 horse v6. Pretty soon we will be telling him to build a dart block, 430 inch stroker with brodix-neal heads and to spin it to 9500 rpm.
For what is sounds like he is looking for, he should either sell his car really cheap to me and get a cobra because the 2v heads on the GT are pretty much hopeless, or find a wrecked 5.0 with a good driveline for a donor vehicle and swap the 5.0, 8.8 and associated hardware into his v6 and if he has any extra money leftover put some afr 165 heads, an edelbroc rpm II intake and a cam on it and have a car that will run low to mid 12s on the motor.