Whip,
If you put a carb on it, it should fit fine. I think part of what Jack is getting at is GT40 heads are not the greatest for a 351W. They are good for a 302, but aren't really adaquate for a 351. If you're on a budget, the Lightning engine will work fine and you can upgrade later. You may be able to get some power out of the Lighning engine by changing the cam also.
Here's an article from Muscle Mustangs & Fast Fords where they got 75hp and 50 ft/lbs torque on an FMS 392 stroker just by taking off the GT40 heads and replacing them with AFR 185s:
http://www.airflowresearch.com/artic...le04/A4-P1.htm
Pete