I would be tempted to replace the gear with the rear of the car placed on ramps or jackstands (not the jack) to reduce (or eliminate) the amount of fluid that comes out during the process. Or park on a hill or curb. Anything to lift the rear of the car. Chock the wheels if on a hill.
Replacing the gear should only take a couple of minutes. You'll probably need a socket/wrench (set), a drain pan, and maybe a pair of needle-nose pliers. I'd have a flashlight ready too. You may have to remove a little electronic assembly which sends vehicle speed information to the cruise control.
If you have a clean container ready for the fluid, you can use that to replenish the transmission. If not, you'll want to have some new fluid. Is it a manual transmission? Or automatic? If it's a manual, it may be a bit of a hassle to refill.. Probably have to have a pump to do it. Or I've done it with a turkey baster (no kidding).
If you get the wrong gear, the worst that will happen is the speedometer may be a bit off.. Not that big of a deal. Check it with a stopwatch over a known distance. Or, if your smart phone has GPS, its speed information is super accurate - assuming it's getting a good signal.
Ford speedometer gears have different colors for different ratios. You could pull the cable (and maybe speed sensor) off and check the color before deciding which gear to get - assuming it has the correct one installed now.
Another option is to figure out the gear ratio of the rear differential, as that (and tire size) determine which speedometer gear you need - less so than engine size.. The differential ratio is usually marked on the housing (pumpkin) - probably something like 3.27:1 or 3.73:1. If it's not marked, you can determine the ratio experimentally.. Chock the front wheels, lift one rear wheel. Put a chalk mark or similar on the drive shaft. Release parking brake and put transmission in neutral (engine off). By hand, turn the lifted tire 10 revolutions while watching and counting the marks that go by on the drive shaft. Divide the number of marks by 10. If you get 33, you have a 3.27:1 axle ratio. If you get 37, you have a 3.73 ratio. 41, then you have 4.10:1. If you're smart with math, you can get away with fewer turns. Maybe even just one turn.. This method might not work if the car has limited slip differential.. You'll find that you can't turn the tire with just one wheel off the ground.
From the ratio, your dealer could tell you which speedometer gear you need - you don't have to buy it from them.
All this assumes it's the plastic gear that's broken. That's probably a safe assumption, as the gear that meshes to it is hardened steel - unlikely to break. It's also slightly possible there'a little metal clip broken in there..
Ah.. Turbo Mustang.. That explains the blown head gasket.
BTW, I drove a car with a disabled speedo for several years.. I was a broke college student and couldn't find/afford the right cable to go in it so I just put a rubber freeze plug in the opening... I did find a factory tachometer for the car for free, so I figured out what speed equated to what engine RPM (in several gears) by timing how long it took to cover a known distance at a steady RPM. A little math and I knew a few key RPMs which would keep me out of trouble with the law.
Good luck! If you made it through a head gasket, this will be easy.