Not a gasket but you may buy and replace the water pump seal for your 91 Ford Mustang GT. You may find the seal in any auto-part Web sites.
There's no certain way the timing belt goes on. You just have to make sure that the belt is tight enough so that you could be sure that the car would run properly.
But check the instructions below and it might help you make the task easier:
1. Begin when your car's engine and cooling system are both cool. Thefirst step is to drain all the oil out of your car's engine. Collectthe oil in a drain pan, so you can reuse it later.
2. Remove your car's fuel tank
,so you can access the radiator if your engine is mounted in such a waythat the radiator is buried beneath the fuel tank. This is notnecessary for all vehicles.
3. Drain all the coolant out of your car and into a second drain pan.
4. Locate the pump seals and loosen them by hand or with a rod. Use ascrewdriver only if it's absolutely necessary, as you might scratch thesurfaces of your pump seals if you're not extremely careful.
5. Make a mental or written note of the direction each seal is facingbefore you remove it. You'll have to orient the replacement seals in anidentical fashion to ensure your water pump works correctly once you'refinished.
6. Remove the seals you want to replace and set them aside.
7. Align the replacement seals in an identical orientation to the oldones, referring to the note you made earlier. With most engines, thespring side of the inner water pump seal faces the engine, and thespring side of the outer water pump seal faces the impeller.
8. Protect the water pump seal springs from rust and corrosion bylubricating them with a little oil before you fasten them into place.
9. Refill your car's cooling system with coolant or antifreeze. It's fineto reuse the coolant you drained earlier if it still has some lifeleft. If not, be sure to dispose of it properly and in accordance withyour state's hazardous-waste guidelines.
10. Reattach your car's fuel tank if you had to remove it in order to access the water pump seals.
11. Refill your car with oil. Make sure that you have appropriate levels ofboth oil and coolant in your car before you drive it again.
Hope these helps and have a great day!