A kit with some lower-profile 90-degree spark plug boots would be ideal, but for now a set of Accel's spark plug boot insulators (P/N 170086) should prevent damage.

Learn How To Install Spark Plug Wire Loom Holders on your vintage Mustang produced by Made 4 You Products - Mustang Monthly Magazine ...

Installing spark plug wire sleeving to help the performance of your engine and protect your wires from exhaust heat.

Also available from ACCEL are these 90-degree Extreme 9000 (PN 9001C) ceramic boot wires for tight applications that call for a sharp turn at the spark plug.