Yes, Ranger 7.5 in gears and the differential will fit in your 7.5 inch Mustang axle.
The same axle shafts fit 7.5 and 8.8 in Mustangs. Only one side from the Ranger is the right length, but if you get two of those, you have the 5 lug swap.
The 7.5 will stand a lot, one of mine has several hundred strip passes, has run lots of 12's, and has had a couple dozen launches on spray with Drag Radials.
It's in an automatic equipped car, and I am careful to lauch deliberately, but it has worked fine.
You can go with the 7.5 until you break it, or round up the money, and or, an 8.8.
I do have three 8.8's ready to go, just in case....