which rod do you think would have better castability a 7'5" mod. fast med. hvy. or a 7' fast med. hvy.? I bring this up because I noticed my 7'3" seems to throw the same bait with the same reel a little further & more accurate than my 7ft.

All things being equal the longer rod will cast further because it has more leverage.

The moderate fast will load up a little more too, increasing distance a tad.

There are so many variables and specs are so subjective, it's impossible to have any hard and fast rule. Then, how are you defining "castability"? Accuracy trumps distance 99% of the time. A shorter, faster tip rod is more accurate in general and a longer, slower rod will cast further. All bets are off though if the rod is severely under or over loaded.

The one you use the most is the one you will cast better with.