Here's a point not mentioned above---and for all I know, material improvements over the past fifty plus years may have made it moot. I ordered my '64 Custom/427 with a 4.11 gear and it worked just fine. But in the early spring, (I got the car in February), we planned a trip from Michigan to Sebring for the 12 Hour Race. Even with the swapped-in 8.20X15 tires on the back, the 4.11 would have been too much of a buzz. I got a 3.50 'pig' and swapped it in. And since I only had a 90 day/3000 mile warranty, I wanted to save something for potential mishaps. I made up a chart of 4th gear engine speeds and car MPHs and then just plugged the speedo hole in the trans. The tach became my speedometer. 3000 RPM was just above 70 MPH, but we decided to fly and in every area we could get away with it, we were running better than 100 MPH as often as possible. This was before I-75 was finished so there were many stretches of two-lane. After the trip was complete, I left the 3.50 gear in the back for a period of time---and I had a clutch failure. Trying to launch a 4000+ pound car from stop lights with the combo of the 2.32 low in the trans with large diameter tires with that gear multiplication actually stripped the face off the disc on one side and I barely made it home.
I'd changed out the T-10 for a big in-'n'-out Toploader which necessitated a truck disc for the 11 1/2 inch clutch. The guys at Bill Brown Ford were willing to look out for me and replaced the disc. And they told me that the 3.50 gear was very hard on the disc. Having a face come off a disc was fairly common under the circumstances.
That was one of the reasons I traded the '64 to my Brother Lon and ordered a new '65 1/2 Mustang 'K' car. Within a few weeks, I'd removed the back seat and put the Shelby deck in its place, replaced the Motorcraft carb with a 715 Holley on the Cobra manifold, put in a Mallory ignition, a Gratiot Cam and a Shelby hood. I removed the 'gills' and replaced them with a pop-riveted cut-to-fit sheet-aluminum panel on each side and installed the rear brake-cooling scoops. All that got the car down to 2880 on the scale at Milan. And then I installed a Paxton VS 59 blower, the GT350 rear brakes, and a set of 4-tube headers.
That car had a 2.32 Top Loader in it from the factory. Street-racing on Telegraph, I could get a running speed set-up that had a 396 Chevy II in second gear while I still had a useful amount of first gear left. I was running a 3.50 rear, but also having a set of tall Pirelli Cinturatos on the back on 15 inch Galaxie Station Wagon wheels. And a slightly smaller set on the front. So with 15 pounds plus of boost, I could just walk away.
Take advantage of planning where you'll be with each component.
KS