After running some numbers through the engine program I have, I decided that 393 is enough. With good heads, mid range roller cam, 750 carb. on a dual plane you can make way over 500 ft/lbs torque. That's enough to set one of these 2200 lb. cars on it's ear. Don't forget that the cubic inches are coming with the same bore size as the 302 and the rod ratios to get a windsor up to 427 are getting pretty tight. So if you gotta stroke it, I'd stay at 393 and call it good.
Another point you may want to consider is that your horsepower seems to want to stay "about the same" as your stroke gets longer. I've read on this forum that if you are going to run on the road courses that you may wish to have more horsepower than torque. If you stroke it you will have the opposite. There are so many variables that the pros and cons are endless. Hard to beat pure gut wrenching torque on the street though.
h dog