Like said earlier, go with the 255. You have a mustang, you won't stop with just heads. You'll find some deal on more go fast goodies, or get beat by an F-body, and want a blue bottle.
yea go with the 255 and just tune it down some,cause for the price of them its worth it to just go with the 255 and droping the tank isnt all that bad it just sucks if it has a full tank .
5.0's have fuel system with a return. Since the price difference between a 110,155,190, and 255 is really small, it doesn't make sense to not get a 255. The pumps install exactly the same and you can't ever flood the engine with too much fuel-at least if your FP regulator is set properly.