The six spark plug well seals are best purchased as part of the Fel-Pro VS50378R set that also includes the two valve cover seals. This set can be bought through AutoZone.com for $71.99, but you may have to look it up as a part for a 1991 SHO since it is not listed for all years of SHOs.
The intake and valve covers must be removed to replace these seals. This raises the question as to whether you have had the valve gaps adjusted in the last 60,000 miles. This would be a good time to do it.
The insulation on the spark plug wires are usually good for 120,000 miles of use. Oil in the plug wells can deteriorate the rubber in the boot seals around the plug. If the wires are less than a few years or 60,000 miles old, you would probably be okay by just cleaning the oil out of the boots and rubber seals the grasp the plugs. If the plugs are close to their 60,000 mile replacement interval, you would be well served by replacing them with the correct Motorcraft AGSP32PP plugs also.
You should always apply dielectric grease to the inside of the plug boots before installing them. This provides a better seal against conductive contaminants and allows the boot to be more easily removed the next time.