What gear ratios are best depends on what ya wanna do with the car. Cruizin or draggin or some of each? The 351C can be built to make plenty of power, the Boss versions could be just plain nasty. They will rev and a lower gear will take advantage of that. How much does your coupe weigh? Prolly not a lot if it is glass. And is it auto or stick? Hard to make recommends without all the info

Pat