I don't Ford ever used either of these ratios on C4 equipped cars. If they're 9" then maybe behind an FMX or a C6. I dunno. I'm running 3.55's in my C4 convertible. Sucky gas mileage but it can really pull out of the hole. I don't think you could tell the difference between 3.50 and 3.55.
I was always under the impression that the 9 inch had the 3.50 and the 8.8 and other ford rear ends used the 3.55. I use the 3.50 with my 351W and AOD and love it for efficiency and performance. I always have to ponder though if I could have gone with the 3.89s...........oh well. John
The difference would have to be something like a thicker ring gear and a thinner pinion gear. But the difference is insignificant. Much like when you bore an engine. You have a minir displacment difference but its really not enough to make everyone run out and bore there engines as a performance incease.